Or it's _really_ close, but the systems are complex enough that they feel they need to QA/cert the games again even if the vast majority of games require no changes.
Do shaders on PS4 games ship precompiled? If so, they'll at least need to either recompile those or create some sort of translator (which certainly isn't impossible). Either way, I'm sure even in an ideal situation, Sony wouldn't want to make outlandishly absolute claims like "perfect backwards-compatibility".
