This could be due to inactive ones, or someone that subbed just because of a particular videos, or it could be also that they bought subscribers form one of those websites that do that service.
I would exclude bots, unless we fall into the dark hole of profit, were youtube draws a line between what is not acceptable, and what is acceptable, looking at the money that comes from that.
Some of them like Buckley do long videos, and frequently: this could wear off your audience, if these minutes are not entertaining the whole time.
Some like Philip DeFranco, do a daily show, but they do interact more with the crowd, and pose themself like "opinion leaders" that makes people on their bandwagons to stick as much as possible, but even his subscribers are not following him entirely.
I strongly suspect that nobody is completely clean at that level, but again: I have no truth in my pocket, and no proof, just plain logic and knowledge.
Plus some like Rjared are being bashed and disliked almost at every video they make, how do they manage to have 20k plus subscribers, and make barely 2k views per video?
Sucks to have that many inactive subs.