hmm,somebody tell me why I received such a reward? why this is a 5 coin,instead of the usual 2,5 
Could be because you minted this block and the one 500 blocks ago. Since you share the reward with whoever minted the one 500 blocks ago, you would collect both rewards at the same time (plus another 2.5 in 500 blocks of course).
That's correct, occasionally you stake a block exactly 500 blocks later and you receive both rewards of 2.5 Sirx at the same time. This is how MPOS is designed to work, it's not an error.