The 12th or 24th word isn't just a standard final word

I just learned something about seed phrases that I hadn't really been aware of before:

Apparently, the 12th or 24th word isn't just a standard final word; rather, it relates to the seed's checksum.

What I found particularly interesting was that the final word isn't necessarily unique. Once the first 11 (or 23) words are fixed, there can actually be multiple valid final words that are formally correct.

In other words: The final word isn't simply "the checksum" - it seems the matter is a bit more complicated than that.

I found this quite fascinating, as I had previously understood it to be much simpler.

Did anyone else here already know this, or have any of you looked into this topic in more detail?