Unlike sum(), the function mean() will always convert to float even when
the result is integer. So conversion happens after summing, which sets
up a user for a nasty surprise.
EDIT: I understand the explanation, but I believe it prioritizes minuscule performance
savings over significant time that will undoubtedly be wasted by
tripping up users with this behaviour. Moreover, this misbehaviour propagates to other functions that rely on mean, like std()