New securely erased drive not empty according to Disk Utility
This is likely coming from one or two things (or both).

Partitioning a disk, any disk, takes up drive space. In the megabyte range it is not even noticeable. In the gigabyte range it can be a substantial byte (sorry...) out of the drive. 

Also computer makers and Drive/RAM makers often calculate bytes differently. O/S companies (Apple, Microsoft, etc...) call 1K 1024 bytes. Drive/RAM makers call 1K 1000 bytes. If you do the math it looks like you "lost" a lot of space when it is just different conventions.

Also formatting a drive will map out bad blocks. So depending on how many bad blocks there are that can reduce the size of the formatted volume as well.