How you explain these performances?

LSI 9260-8i

Case 1
5x Seagate Constellation ES 2.0TB (+1x dedicated hot spare)
RAID 5 - Strip Size 256KB - Write Back enabled

read/write performance: 300~325MB/s
no way to get more

Case 2
6x Seagate Constellation ES 2.0TB
RAID 5 - Strip Size 256KB - Write Back enabled

read/write performance: 600~640MB/s

For testing I'm using dd in this way
write: dd bs=1M count=1234 if=/dev/zero of=/mount/point conv=fdatasync
read: dd bs=1M count=1234...

How you explain these performances?