At times, the number of data pages in a partition can be balanced, while the number of data pages on the devices in a segment becomes unbalanced.
You can check the free space on devices with sp_helpsegment. This portion of the sp_helpsegment report for the same table shown in the sp_helpartition example above shows that the distribution of pages on the devices remains balanced:
device size free_pages ---------------------- --------------- ----------- pubtune_detail01 15.0MB 4480 pubtune_detail02 15.0MB 4872 pubtune_detail03 15.0MB 4760 pubtune_detail04 15.0MB 4864 pubtune_detail05 15.0MB 4696 pubtune_detail06 15.0MB 4752 pubtune_detail07 15.0MB 4752 pubtune_detail08 15.0MB 4816 pubtune_detail09 15.0MB 4928