This is because we've been mining some very fast coin during the last hour with new blocks coming every 1-5 seconds.

Don't worry with the rejects, we only mine such coins if it's profitable. For example: If you mine 200% profitable coin with 10% rejects you still have 0.9*200% = 180%. If you mine 150% profitable coin with 2% rejects you have 0.98*150% = 147%. What would you choose?