Conflicting size info Disk management and Shrink fonction

On this laptop with 2 HD in RAID 0 configuration, Vista Disk Management
reports:
1)a 10 GB recovery hidden primary parition and
2)a 288 GB C: primary NTFS partition with 265 GB free space.

Yet, if I right click C: to add an other partition using Shrink, the popup
windows lists (in rounded value) for C:
size befoe: 295 GB
size available to shrink: 92 GB
Amount of space to shrink: 92 GB
total size after shrink: 263 GB

Is this conflict between free size (265 GB) and amount of space Shrink
reports available(92 GB) a result of the Raid configuration ?
I did cleanup and defragmented C:

Thanks in advance.

--
Alain Caillet