So, 9 months is a bit of a shorthand used in common discourse -- I doubt that the real life average is perfectly that -- but looking for specific numbers, it sounds like the 40 week number is the average time from the last pre-conception menstral period, rather then the average time from conception.

https://en.wikipedia.org/wiki/Pregnancy

Duration: ~40 weeks from the last menstrual period (38 weeks after conception)

So, working through this, Halloween night wouldn't have been the time that Lily Potter had her last menstrual period, but rather the point in time that she and Satan were actually getting things in motion.

That says that the actual average time from conception to birth is a week less, so one could maybe argue that it should be July 24 or that Harry was a little off average.