Good catch, I hadn't seen that population drop! Your explanation makes sense to me.

I only noticed the retraining because I was watching those garrison unit strengths in order to time my attack. It helps that I'm in a campaign where I'm using lots of spies - so they can "persuade" other cities to throw off their overlords and return to their Hellenic roots :) Now that I've seen retraining happen once I'll look for it again.

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