g++ 4.4 implemented several key parts of C++11, including notably rvalue references, and adapted libstdc++ to use rvalue references in C++11 mode. However, the committee had to make major revisions to rvalue references subsequent to this implementation, to the point that you can't use libstdc++ 4.4's header files (such as <vector>) with a compliant C++11 compiler. So when you try to use newer clang (which prefers to use system libstdc++ for ABI reasons) on systems with 4.4 installed (because conservative IT departments), the result is lots and lots of pain.

Furthermore, it absolutely is the case that newer versions of compilers will interpret old language standards differently than older versions of the compiler. You don't notice it for the most part because the changes tend to revolve around pretty obscure language wordings involving combinations of features that most people won't hit. Compilers are going to try hard not to care about language versions past the frontend of the compiler--if the specification change lies entirely in the middle or backend, then that change is likely to be retroactively applied to past versions because otherwise the plumbing necessary is too difficult.