The exchange rate between bitcoin and USD fluctuates (and, on average, has been going up a lot). In 2011, the dollar value of a single bitcoin was far less than it is today. I have no idea what the value of a bitcoin was when you bought in 2011, but suppose 1 BTC was worth $0.01. For $6.42 (ignoring fees) you could have bought 642 BTC.
Fast forward to today, where 1 BTC is worth about $2400, then your 642 BTC would be worth $1540800.
When Bitcoin started, it was worthless. Anecdotally, the first bitcoin transaction for a real world item was for two pizzas for 10,000 BTC. Apparently that was worth $41 at the time, so 1 BTC was worth $0.0041. Back then, nobody had any idea how much 1 BTC should be worth.
It seems rather unlikely that a bug has caused you to unknowingly acquire bitcoins without your involvement. Instead, it seems plausible that you bought in very early when the price was quite low, and now have an asset that is worth quite a bit more than what you paid for it.