My understanding is that there's something called a race condition or a fork, but over 50% of network has to reach consensus or the bogus branch will wither and die... ? Too simple an explanation I'm sure but it might be related to what your observing... Apparently your miner could start mining at any block, maybe due to bad time settings? IDK... 

 That was my initial thought, but the blocks were numbered sequentially. If it were a fork, I'd expect there to be two competing blocks with the same number.

Based on the comments and links in the replies, it seems like the most likely explanation is that someone's clock was significantly off.