That's the part I'm not sure about myself. I'm just psychoanalyzing a person I've never met, which can't really be done beyond speculation. Typically lying about something like this would be to try to gain something from it. In this case, how could he anonymously gain because no one is going to find his books and check them out if they don't know who he is.
Second, one could argue that he's lying so that people don't look up his books and discover that he's really only sold a few dozen copies. That's possible.
It's also possible that there could be other motivations: attention, for instance. Even anonymously, maybe getting his fifteen minutes of internet fame makes him feel special. It's similar to people going online and making up insanely ridiculous stories about crazy things that happen to them or claim to have a terminal disease just for attention (I've met people who have done this). Someone anonymously claiming to have sold thousands of copies
could fall into that category. I'm not saying he does because I really don't know. I'm just randomly speculating on potential motivations.
The thing is, if he's really selling this well shouldn't it be obvious? Even if it's scattered over numerous books, shouldn't at least one of those be in a top sellers list? And if he really has that many books out it would be easy to figure out who he is just by clicking on the author's name.
I think that's what makes me more skeptical than anything. I feel like even if he wasn't giving his name, it should be showing up just in fan lists or kindle bestsellers or in some way gaining some steam even without his statement, or at the very least that someone would have been able to check out the list and say "Oh, it's obviously this guy because he's the only one on the bestseller lists with 80 books."
Is it possible that with so many he could just be selling four or five copies of each and still make that amount? It's too early in the morning for me to do math.