We’re still trying to figure it out ourselves. Believe me. You’re not alone.
The humor in this illustrates why supposed “technical” explanations (or “excuses”), are not helpful to end-users.
The “ZFS magic math” is a meme in of itself. It’s frustrating and confusing to the user, which I feel your very frustration and confusion. It’s not acceptable for NAS users.
See this thread, which already has 75 replies. It goes to show you how nonsensical this is. End-users are left to scratch their head and wonder “how much space do I really have?”
For the record: I side with the users, not the developers, when it comes to these issues. Even if something “works”, it has to make sense to the person actually using it. ZFS “capacity” is still not intuitive when it comes to RAIDZ and RAIDZ expansion. (Case in point: your opening post.)
Can you actually fit 24 TiB worth of data in your pool of four vdevs, which should yield 16 TiB? Who knows! It certainty acts like you can… but you know it’s lying, since the math doesn’t add up.