different PD number

Hi amigo:
My OS:FreeBSD 9.3-Release-p33
RAID card:MegaRaid SAS 8708ELP
HD:SAS 146G*4 Raid5

I see the number in the MegaRaid card bios is: PD8, PD9, PD15, PD16
but the number in the OS message is PD8, PD9, PD10, PD13
why the PD number are different?

it looks very very similar to what happens when there's a serious problem with something.
kernel: mfi0: 45356 (509619603s/0x0002/info) - Unexpected sense: PD 10(e1/s5)
kernel: mfi0: 45355 (509619603s/0x0002/WARN) - Predictive failure: PD 10(e1/s5)

Thank you in advance.