I haven't done the math, but I believe it's because of two points: 1 - The "difficulty" of mining a block is based on how long it's been since the last block w

I haven't done the math, but I believe it's because of two points: 1 - The "difficulty" of mining a block is based on how long it's been since the last block was mined, with a drop off in difficulty at 9 seconds 2 - If two blocks "tie", the the block that was more difficult to mine is the winner, and takes the rewards. --- So essentially, it's a way of breaking ties in their favor - reliably - by cheating a smidge on the timestamps they send for their blocks.