I've been growing a lot of lettuce recently to get my familiarity with the crop up, and I've noticed a strange trend. When I ignore the crop, I get 50+ heads of lettuce per harvest, and usually much higher. One time, I'm pretty sure I even got 111 of them but I clicked off too fast to grab a screenshot. When I manually tend to the crop, I seem to get a reasonable, expected yield. However, when I use the Gardener to tend to my crops for me, I get the WORST yield. When he is actively tending to them, I'm only getting between 10 - 25 heads of lettuce per crop. I just picked up on this trend this morning, so I'm going to return here with more concrete data and screenshots as I harvest my lettuce over the next day or so. Something weird is definitely going on with the lettuce crops though - I don't know if this is exclusive to lettuce or if people have noticed this with other crops as well.
In case it matters, I've been level 3 and 4 familiarity with lettuce when I started to take notice of the harvest yield.