Why does Stellar Partition Manager insist on keeping 128 MB "free space" on the medium?
The reason to leave 128 MB after each partition is given in an Apple's Tech Note: 


  Note: We leave free space after each partition to make it easier for future system software to manipulate the partition map in ways that we can't anticipate currently.


In reality they adhere to their own policy only in special cases:


Formatting a non-system disk to one or more partitions
creating a Boot Camp partition


On a system disk there neither are 128 MB free spaces between partitions (EFI – system partition / system partition – Recovery HD) nor at the end.