Momentus xt (hybrid) bad performance...

hi...i have a system with intel i7 870,gigabyte p55a-ud4 ,8gb ocz3f1600LV4GK and 4 hard disks.
2 seagate barracuda st31000528as
1 seagate momentus xt st93205620 as (hybrid)-system disk
1 st 95005620as(hybrid)
my graphic card is an ati radeon hd 4550

the problem is that whenever i do some benchmark test ,the performance of the momentus (read-write),especially the write is not the performance i expected....it is almost the same or worst than common barracudas....does anybody knows why this happens??i have windows xp 32 sp3.

thanks,,,,