Each build backend can have its own extensions to the standard packaging metadata. In practice this is common because Python packaging wasn’t designed to solve language ecosystem problems, but to make the union of maintainers, Linux distros, labs, and everybody else who uses Python somewhat happy and able to preserve existing flows.

This is bad for obvious reasons. But it’s not the same thing as Python packaging not being standardized; I would say that Python packaging is as standardized as its user base is willing to stomach. That willingness has grown over the years, to the immense credit of a large group of users who otherwise don’t agree on much.