How does the AI pick army composition?

[Chris1959]

Just wondering how the AI forces "pick" themselves, as sometimes stacks seem quite well balanced othertimes an odd mix of elite or dross troops.
Is it me or despite being renowned for their cavalry Gallic armies in 1.1 seem to field very little, in my current campaign at 170BC and I don't think I've seen any Gallic cavalry apart from FM bodyguards. The exception being the start up rebel forces.
I try to stick to a sort of "historical" force as recommended by some in the forums.