This is the way the Germans formed their Armies in the GREAT WAR-
1-6 & 7-8 from the 7 Army Inspectorates of Prussia, Saxony, Baden, Wurtemberg etc.
6th army from the Bavarian Armies units.
9th army from some elements of the 8th army and using reserves and ersatz raised in Germany, similarly the 10th.
The 1st, 2nd and 3rd armies as also the other armies gave up elements like- Guards, Guard Reserves etc to form the 11th and 12th armies.
That is why you don't see them in start; as they were sent to East Prussia in September.
Now the remaining armies like 14,15,16,17 were formed by breaking each division containing 4 regiments pre-war into triangular divisions and using the extra regiments to form new divisions and new armies. Also several good divisions like- Prussians, Guards etc were moved east and the Reservists raised in War time were sent West and used for Sentry posting.
Divisions were classified into A,B & C - A good for attack and defense, B good for defense and ok for attack, C only good for guard duty and holding trenches till reinforcements come.
This meant that the EINGREIFF or counter-attack divisions as designated on the Western Front by Col. Lossberg and Col. Bauer who designed the Defense-in-depth system in 1915-1916 were mostly A class and former 'Regular Army' units.