Some of the books GW have released contradict later ones (Dan Abnett comments on the canon not being coherent at the start of Fire & Steel), and GW have never issued a single applies to all situations rule of interpreting, so you have two choices:
- The latest book overrules contradictions in previous ones
- The contradictions represent rumour and folklore being more common than objective truth in a normal pre-industrial culture, so one where there is a god of change won't have any consistent history.