With groove measurement, whenever an odd size shows up which appears to be a long way out of spec, I reckon it's a good idea to start again, re-slug both bores, and re-measure.
You also need to be 100% certain that the slug is actually in full contact with the groove diameter, and that the micrometer measurement is taken accurately.

If they really do turn out to be .4035", personally I'd be getting a reducing die made for .408" bullets, to take them down around .404" to .405".

Firstly though, I'd want to be dead sure that the groove measurement is correct.