Interesting fact about Maes Howe is that recent researches found evidence for at least a pre-existant standing stone, so the idea is that Maes Howe was built on the site of an henge. Could it be that the mound was mostly finished before the stone structure was fully under way ? Nah ! Second thought; if a Neolithic 'tourist' took the mound idea from Maes Howe maybe the site was only seen from afar, too sacred to allow just anyone near. So copied only a mound.
If rather than being homologous Maes Howe and Silbury are analogous could it be the lack of decent structures found within the latter is that Silbury Hill too has been built over a pre-existing henge monument, only without any other mods ?