[throwaway0a5e]

You forgot to multiply by the likelihood that the timer (or some other "too small to take the baby out" distraction) goes off mid bath.

When you say "the vast majority of parents appear to take 'never' to heart" I can't help but suspect that missing factor is doing a lot of the heavy lifting.

[zamfi]

I assumed that it happened once per bath, but this is very context- and parent-dependent too.

If you don't perceive of a 1-minute absence as a problem, presumably you are much more likely to step out of the bathroom.