Vista - Conflict disk management - Shrink popup window

On this VAIO VGN-AR41S with two 160 GB HD in a RAID 0 configuration, the
disk management utility reports:
- Healthy (EISA configuration) hidden factory restore partition: 9.78 Gb
- C: NTFS capacity 288.32 GB free: 265.86 GB (92%)

Yet, right clicking the C: drive "strip picture" and selecting Shrink. the
popup windows reads: size before: 295236 MB
available shrink space: 91973 Mb
amount of space to shrink: 91973 MB
total size after shrink: 263263 Mb

Why the size discrepancy between the two reports from the same tool? How
come I could only shrink 91.973 GB of a primary partition which has 265.86
GB free to turn them into an other partition (D?

I notice too that nowhere does it says this is RAID 0 (striping) disk
configuration. Am I missing something?

Thanks in advance
--
Alain Caillet