Notice how in the first image, there's a wild spider, 3 tame spiders, then 2 wild ones.
Now, normally, if I feed a spider, I would tame 3 at once. So I fed the one on the left, but only THAT spider got tamed. The rest are still wild.
1. Have some friendly spiders
2. Feed a wild spider while a friendly spider is closer to it than another wild spider
3. Notice how you tame less than 3 spiders per meat despite the fact that there are enough nearby spiders