Unless you’re carefully documenting their stats and the raises they get each level, the only reasonable thing to do is assume that’s it’s confirmation bias. With all of the percentages and chances involved with leveling up thralls, it’s pretty much impossible to be sure unless you’re truly tracking everything that happens. And of course this isn’t directed at you personally, confirmation bias is really hard to overcome, it’s only possible with careful data tracking.

I have a spreadsheet for the 20 thralls I’ve leveled up in the last couple of months and I don’t see anything unusual but to be fair I didn’t track every single level and I didn’t specifically track which ones had leveling food and which ones didn’t. I just recorded their stats at Lvl 0 and then their stats at Lvl 20.

People “notice” lots of things but that doesn’t mean they’re true, the only way to be sure is to carefully gather data on all of them as you level them up.