Your math only makes sense if you’re assuming they got all their customers year one and have had no growth since. Who’s to say the donations last year weren’t $100,000 putting the estimated profits at $1.2m (assuming all months are roughly equal)?
To answer the rhetorical question: I can tell you that. I wish, though. ;-)
Reality is somewhere between those two hypotheticals. The past few years, my now-2.5-year-old has been taking a lot of my attention and I haven't really been focused on Tarsnap's growth.
$300,000/year for 14 years is $4.2m
If you’re making $1m in profit after 14 years it doesn’t take long to catch up even if your first few years were slow. You also are sitting on a business you could likely sell for $5-10m on top of that.