As a C++ developer you can think of Python variables as pointers.
Thus when you write spam = 100, this means that you “assign the pointer”, which was previously pointing to the object 42, to point to the object 100.
Earlier on, cheese was assigned to point to the same object as spam pointed to, which happened to be 42 at that time. Since you have not modified cheese, it still points to 42.
Immutability has nothing to do with it in this case, since pointer assignment does not change anything about the object being pointed to.