The chance to fail 4 or 5 times in 5 gathers with 85% chance is 1 in 395. Since people probably gather considerably more than 395 times if you do a lot of gathering, it'd actually be pretty weird to not fail 4 times in 5 gathers at some point.

One of the really simple things about probabilities is that if something happnens 1 in 1000, but you're doing the activity 1000 (or more) times, it's very likely you'll see a 1 in 1000 event at some point. There's an experiment where a teacher had students roll a 6 sided dice 100 times and write down the results, and have other students just fake 100 rolls, and the teacher was able to identify all the fake rolls because they didn't have stuff like rolling the same number 3 times, even though there's a 94% chance you ought to roll the same number 3 times in a row in 100 rolls.