Hi Jim,
My brain is starting to give off smoke
In the real world depots are installations for storing large amounts of supply for redistribution to other locations. The amount of what is stored depends upon what has been reported as necessary by the organizations being serviced.
In-game they work much the same way with the exception that the organizations do not actively (from the player's stand point [thank goodness]) make requests for supply. That happens in the supply phase of the game at or near the beginning of each turn.
So land units 'report' that they need X amount of which supply which (and this is where my knowledge drops off) is then pulled from the closest supply source (supply train/ships, depots, cities). However the descriptions of supply distribution often speaks of 'pushing' supplies from one location with supplies to the next. I am guessing that locations with or without depots that seam to accumulate large amounts of supply do so coincidentally because more supply is being 'pushed' through them than is being received by the military units actually 'requesting' that supply. This I assume is how the supply-algorithm works.
Example of how my understanding of supply distribution works:
X is a unit with a supply train of 4 elements. The supply train can carry 80 each GS (General Supply) and Ammo (Ammunition Supply) and 'requests' what it lacks each turn from the closest supply source(s?) (I can't remember ever hearing if supply is pushed/pulled from more than one location to a single unit). X uses 10 GS per turn and can inherently carry 20 GS itself.
C is a depot in the field one region distanced from X.
B is a level 1 city with a depot 3 regions distanced from C and 4 regions distanced from X.
A is a level 5 city which produces each 500 GS and Ammo. It is 3 regions distanced from B, 6 regions distanced from A and 7 regions distanced from X.
A and B are connected by a rail-line of the same length as their distance from each other.
At the beginning of the supply phase on day one of each turn (see the AGE-Wiki/Supply link below) X uses 10 GS and thus is 10 GS down from it's full-supply. This is taken from the supply train in it's stack. Now the supply train is lacking 10 GS. This is 'requested' from the depot in C.
C pushes or X(supply train) pulls 10 GS from the depot in C.
C, because it is having supply 'pulled' from it, 'requests' not 10 but 30 GS from B. (I'm really just guessing at this value, because I've never even tried to figure it out, but it seams to me that locations in the supply-chain are always getting a much larger amount of supply than being consumed at the end-unit(s)).
B, now down 30 GS, 'requests' not 30 GS but 90 GS from A, which is then 'pulled' to B.
A now has 410 GS left from what it produces each turn. Whether any of this is also 'pushed' along to B and C I have no idea.
However, if in my theoretical example above, supply is only 'pulled' in the amount needed by the end-units, then the rest of the supply in the supply-chain being built-up in each location in supply-chain (my observations) must be being 'pushed' there by that location 'remembering' that supply was being 'pulled' through that location, thus allowing more supply to be transported into and through all locations in the supply-chain than actually used by the end-units.
Jim, carrying this example of land-supply over to naval-supply, this would pretty much explain how your observations come about, with regards to supply being accumulated in forts with depots where units are present.
But in Gray's
Supply Primer and in the
AGE-Wiki/Supply
it is also described that locations with limited supply capacity (how much they can contain I am assuming, as it is not really spelled out) will not allow more supply to be 'pushed'/'pulled' though it per 'push'/'pull' iteration; thus a bottle-neck is in the supply-chain. Here it is also described that this bottle-neck can be eliminated by building a depot in locations with limited capacity.
So what is the supply-chain using naval-supply? My assumption is, from major cities with large supply-stockpiles and large harbors adjacent to a coastal region (New York, Boston) to the Atlantic Shipping Box to a Fort or City-Harbor with or without a depot also adjacent to a coastal region. It doesn't appear to me that it is possible to build a chain of forts or cities along the coast which would work as a supply chain. Each is either a supply source or a supply consumer and only transport supply inland but not to the next fort or city along the coast.
How far supply is transported is also described in the above links, but only for land supply. Naval supply is ignored in these sources. But from my observations the same. That is, the number of ticks for supply to reach a locations is calculated referencing weather and other factors to see whether a supply chain exists. There must also be some factor about how much supply is transported, because the descriptions only describe the chain as being completed or not, but not that supply transportation is being slowed or hampered by adverse affects.
From all my readings and experience, the bottle-necks I've experiences in the Gulf of Mexico occur because of bad weather, and an increase in supply usage in the area. Why the affects are sometimes so grave or don't seam to dissipate during seasonal good weather is beyond my experience or understanding. Maybe as Carrington suggests, something changed at some time in the coding which reduced the actual naval-supply-capacity over larger distances, but I just don't know.