[xorcist]

That's only because we have a weird method for calculating pregnancies, for historical reasons.

A human pregnancy is around 266 days +- 16. The reason for 280 days or 9 months is because that measures the time span since the last menstruation, not since conception.

(I was personally a bit surprised by this when expecting our first child. It's one of those things you apparently know about but still use "9 months" as a bit of a fixed expression.)