In macOS Sierra 10.12.6, in Disk Utility, my MX300 525GB model is showing as actually having 525.11 GB of available space. This seems much higher than it should be. I have the same model SSD in another Windows PC and usable space is only 488, as expected, and as I've read it should be. Is this a problem?
Windows still treats 1KB as 1024 despite the standard for 1 kilobyte being defined as 1000 since 1998. Drive manufacturers, at least some Linux based operating systems (and presumably Apple based upon what you are saying!) treat 1K as 1000.