Because virtually everyone that I know that has has babies in the past few years has had them 'a few days/a week early'. All 3 of mine were late but no one else ever seems to go over due.
DH's friend's wife has just had her third and told me confidently during pregnancy 'Oh mine are always early, this one will be too' and lo and behold, it was born a week early.
Another woman that I know has just had her second baby, and again, as with her first, her second was 'a few days early'.
I've heard before that famous people give a fake due date so that it looks like they've had their baby early. AIBU to wonder if 'real' people do this too?