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.

How old is the drive?

Flash memory has limited write cycles, and it will slowly change into a read-only device. If you have been writing stuff to the drive for a few years it is quite possible some blocks have reached their write limit and the controller has marked them as unusable. Cheaper drive -> fewer cycles.

Also note that the one-pass-of-zeros secure erase method doesn't really work on solid state media, and it's another hit on the drive's write cycles.