It may not be fundamentally different but it may only use a subset of features (e.g., no exceptions in C++ code) and/or target a subset of platforms. Without pointing fingers, I am sitting on (what looks to be) a codegen bug in Clang that is not of a high priority I am pretty sure due to the above reasons.

This is not to say that I am not appreciative of all the work Google/Apple/etc engineers do in LLVM (I will be eternally grateful for the Clang targeting MSVC work).