I didn’t actually assume the F really stood for Fluctuating, but that is me.
These issues are in play for me constantly, not necessary because of snapshots, though those too, but because I’m more than happy to use and abuse my favorite APFS capability, copy-on-write, which enables me to, say, think nothing about duplicating a VM or a sparse bundle or an iMovie library or a Photos library before making changes to it, since those copies take up no additional space. It’s so convenient! I can operate safely without doubling the space I’m using! But it also means that disk math is impossible, because of course it appears that I have hundreds more GB in use than I actually do.
The part that feels frustrating to me is that, in theory, a utility could analyze APFS data structures and theoretically figure this stuff out – I mean, after all, the computer has to know at some level what’s a lightweight duplicate or snapshot versus a block not otherwise represented. But I’m unaware of any tool, even seven years later, that works beneath the APFS file level, other than, of course, fsck_apfs. I don’t know if this is because Apple hasn’t released sufficient technical detail about APFS to make it possible, or whether it’s just because it’s so complex that no one wants to make it, but it’s a bummer. We could use it.
Also problematic is that if you make use of these APFS features, they’re part of the largely non-copyable container they’re in, meaning what fits on one computer may well not fit on another drive of the same size. This was a problem for me when cloning one computer that had about 1.9 TB used on a 2 TB drive – Migration Assistant just refused. I ended up using the Legacy Clone feature of Carbon Copy Cloner, which utilizes a somewhat unreliable Apple-provided underlying container copying mechanism, but it’s hardly something I’d want to depend on. I’d at least think Migration Assistant ought to have the capability of figuring out what is real space occupied when moving from machine to machine, but, no.
As for whether it’s possible to intentionally purge purgeable space – DaisyDisk does this, and in general has long been a tremendously excellent tool for managing disk space. It’s right up there with Carbon Copy Cloner as far as essential Mac tools go for me. I don’t think it has any way of knowing about what’s an APFS copy-on-write clone and what isn’t, but it at least helps me figure out how much space is hidden (meaning, snapshots and other stuff outside the immediate file system), and what the big fish are – or at least big fish candidates – when I’m looking to clear some space up.