Unstable output of FFSB (Filesystem benchmark)

I'm trying to benchmark a RAID-6 server using FFSB and the result is really unstable to detect the pattern.

Scenario -1:

Block-size - 65536 (64KB)
Number of threads - 10
Number of files - 10
Min and Max filesize - 400m
Read size - 400m
Direct IO - on
Random-read=off

Read throughput - 420MB/s (This is expected)

Scenario - 2:

Block-size - 65537 (64KB + 1 Byte)
Number of threads - 10
Number of files - 10
Min and Max filesize - 400m
Read size - 400m
Direct IO - on
Random-read=off

Read throughput - 30GB/s (this is not)

Can anyone explain this behaviour?