How do your profits go back in time???
How do
you win at satoshidice if you don't have a time machine?
But seriously, since Bitcoin is distributed, and trusts in no central authority at all, the network doesn't know what time it is. We can't rely on a central time server to tell us. So the nodes tell each other what time they think it is, and each node works out how 'wrong' its own clock is based on how different it if from the times as reported by the nodes it talks to.
When a new block is received, we don't throw it away if its timestamp is earlier than the timestamp of the previous block. Maybe the previous block's timestamp was wrong, and the new block's is right. So what happens is we look back about 11 blocks, and look back 1 block, and take the average of those two timestamps. If the new block's timestamp is later than that average, it's accepted. This means that sometimes a block's timestamp is older than the previous block's timestamp, which is probably what you're seeing in the graph.