Theseus and Newt are twins, and occasionally switch identities. (Say, when one of them needs to avoid a particularly boring meeting). Newt, due to poor timing, accidentally ends up pretending to be Theseus while his brother was supposed to be on the clock - meaning that Newt Scamander, magizoologist, is effectively Head Auror of the Ministry of Magic until his brother manages to get back from his emergency holiday shopping.

+ Gramander. Somehow.
++ Graves finds out, and finds that the idea of Newt in auror robes - and acting as a /competent/ Director Scamander - is a kink he never knew he was into.