I understood your question to be: "Why are these variables called PUID and PGID instead of UID and GID"? In fact they could be named anything at all. Neither Docker nor Linux care. It's the application that decides what to do with them.
Further searching revealed that this is merely a convention, and some applications do their own thing. For instance, one application uses the environment variables MAYAN_USER_UID and MAYAN_USER_GID respectively.
