I don't think it's so much about consistency, it's rather about being "better".
But Nadal's peak was reached in 2008 and he has physically declined since then, Murray's peak was reached last year and nothing says that he has reached his final peak yet imo as he rather improved in last years and has a lot of weapons in his arsenal and typically those players need time to find how to fully use them well in the end.
Anyway, I understand these arguments but I think Murray is very underrated in this forum, and Djokovic, who imo played his best tennis last year in the end (from Beijing) and the very-beginning of the year (in Australia), had incredibly tough matches against Murray in those "at his best" period last year.
Murray is dangerous enough in the semis or finals, however between Djokovic, Nadal and Murray to have an early, i.e. "shock" exit from a masters, it's been usually Murray so far.