For a PR pblock, as long as you do not select IOB type, then the SLICE column adjacent to the IOB will be snapped out of the Pblock's derived ranges.
This behavior is not extending to its child Pblock, and the SLICE column adjacent to the IOB is still reserved for the child pblock.
The following error message will be reported for the mismatch of parent and child pblock:
To resolve this issue, add the missing ranges to the parent or child Pblock.
In Vivado 2015.3, the child pblock will inherit the property from the PR parent pblock and the SNAPPING will behave the same for both Pblocks.