Why does laying farms earlier with boar-gathering vils cause me to have more food by the time I click up to castle?

I've been practicing a no-deer pocket build 27->29 pop and I'm a bit confounded by something I can't figure out:

If I keep all my boar/sheep vils on sheep and boar until they're done and THEN have a few of them make farms, I have substantially less food (about 150-200 less) than if I take a few off the sheep/boar and start the farms before the sheep and boar are done.

This make no fucking sense to me. Either way the same number of vils are working on food for the same amount of time, no? How am I ending up with more?

And we're only talking 3-4 farms here.