So, basically we have this city here:
According to the manual, 6 supplies are generated per city level, according to the structures file, it's 8. The unit present consumes three supply points every turn. The difference between the 163 points of supply sent and the 108 received is 55. Where then does the 48 come from? Am I even correct in my assumption that the city would create round about 60 supply itself (8*5*1,5 for 100% loyalty and a bit more for National Morale over 100)? It's after all kinda important to calculate the supply production and forwarding of a particular city to plan ahead how large an army I can support in a respective area.
So 4 City Levels * 8 Supplies = 32, then multiplied by 1,5 for 100% loyalty = 48 supplies like on the screen. 