is it possible that 1024 and 2048 block size read speed is faster than 4096 and 8192 block?

I'm using lsi 9361-8i with RAID 10 , with 8 x Kingston E50 250G.

result:

• 1024 = Write: 2,251 MB/s Read: 2,625 MB/s
• 2048 = Write: 2,141 MB/s Read: 3,672 MB/s
• 4096 = Write: 2,147 MB/s Read: 231 MB/s
• 8192 = Write: 2,147 MB/s Read: 442 MB/s

is there any possible?

and below is the reading when i simply want to test out the RAID 10 function and disaster test by taking out one of the 250G harddisk. the result is different like below: Result:

• 1024 = Write: 825 MB/s Read: 1,139 MB/s
• 2048 = Write: 797 MB/s Read: 1,312 MB/s
• 4096 = Write: 911 MB/s Read: 1,342 MB/s
• 8192 = Write: 786 MB/s Read: 1,204 MB/s

and the result for 4096 and 8192block are different? can any one explain to me is it normal? or I need to do some tuning/configuration? will it affect my host linux performance?