C/C++ "undefined behavior" means your program is wrong and not within the spec, not that different compiler implementations might differ. There is no scenario in which any code with UB is valid, regardless of what compiler implementation you use. That's very different. (And IMHO a horrible feature of the spec, but it's there for historical reasons.)

C/C++ spec does have some "implementation defined behavior", which is where you can't rely on what will happen unless you know what compiler you're targeting. They are explicitly listed out as such, and not that common.