I came to know through this answer that:

Signed overflow due to computation is still undefined behavior in C++20 while Signed overflow due to conversion is well defined in C++20(which was implementation defined for Pre-C++20).

And this change in the signed overflow due to conversion is because that from C++20 compilers are required use 2's complement.

My question is:

If compilers are required to use 2's complement from C++20, then why isn't signed overflow due to computation well-defined just like for signed overflow due to conversion?

That is, why(how) is there a difference between overflow due to computation and overflow due to conversion. Essentially, why these two kinds of overflows are treated differently.