Automatic battles = faster XP gain?

[Ibrahim]

Ibrahim, my Roman campaign on VH/M saw me fight 3-5 battles against fullstack Luso and Pontos+AS armies with two main armies. Often with 1-3 Heroic each turn, yet in most of these only 1-3 units would gain XP, and often not the ones who did the hardest fighting. Sometimes the logic of it puzzles me.

Toxotai Kretakoi with 1-3 gold chevrons kick proverbial though...

now that I just noticed; I must say that that makes no sense.