when you format a 256GB drive and find out that you don’t actually have 256GB

Most of the time you have at least 256GB. It’s just you 256GB=238.4GiB, and windows reports GiB but calls them GB. You wouldn’t have that problem in Mac OS that counts GB properly, or gnome that counts GiB and calls them GiB.

(This is ignoring the few MB that takes to format a drive, but that’s also space on the disk and you’re the one choosing to partition and format the drive. If you dumped a file straight into the drive you’d get that back, but it would be kind of inconvenient)