This is a great example of "Harmonic Rhythm". Just like single note lines that have strong beats and weak beats. Our ears perceive progressions the same.In a typical four bar phrase the first down beat is the strongest followed by the 3rd bars downbeat. Then bars 2 and four. If you bend your self to hear the prog. G/G/D/C there would be no doubt. alot of the ambiguity is based on that the home is not on the one. Then there is all the Rock and Roll Rub going on. Some guys soloing with some changes (first solo D triad right out front). Opposed to the Piano Solo which is clearly in G and sounding the most "in". The bass laying into the F natural on the outro,while using the F#'s as the kicks. This is where so much musicality and fun is. Sure it may lead to some confusion in the analysis. Always keep analysis is not meant for anything other and creating a vocabulary for chatting about what we hear.