How large is the mytab table? Run
sp_spaceused mytab, 1
A possible factor is that REORG REBUILD, CREATE INDEX, and minimally-logged BCP use a special allocation method that can't use the first extent (which only has 7 usable pages instead of 8 as the first page contains the allocation page that controls the allocation unit). So these commands can get 1105 errors even when there is some free space still on the segment.
-bret