It is a branch of knowledge, no less no more. The science of religion is relative, that is relative to the presuppositions. The science of religion is age-bound, because presuppositions are. Revealed religion itself may be true and free from contradictions, but science of religion is not necessarily so. Religion may be perfect or comprehensive, but not so for the science of religion.
Religion is divine, but interpretation of it is human in and out.
That is the story of religion. All this implies that religion is always surrounded by a host of contemporaneous data and deliberations, in constant give and take with them, the interpretation of which remains constant so long as these external elements are constant, and once they change, the change will be reflected in the understanding of religion as well. Therefore it is not because of the conspiracy or aberration of mind or illegitimate manipulation or extravagant interpretations that the science of religion changes. Rather, it is the natural product of the evolution of human understanding in the non-religious fields and contexts that forces the religion to be comprehended differently. And as mentioned above, external factors are responsible not only for the change, but also for the constancy of religious interpretation during ages.