As you can notice in this graph, even at peak 40 weeks gestation time before inductions and c sections were more common, only up to 40% of births occur at 40 weeks.
Plus, sperm can easily last 5 days untill fertilization.
And sometimes ovulation can occur a bit earlier than 14 days, which is why the time after a period cannot be considered safe in terms of avoiding conception.
It is quite possible that some people here were indeed conceived at events, even if they were born 40 weeks after that event. Less likely if they were born on their due date, as the mums would have been starting their period then.
Also pointing out that 40 weeks is 280 days. I.e. 9,20 months at an average of 30,42 days per month. That means 9 months and 6 days, or 9 months and one week approximately.
Studies like this suggest the median can be more like 283 days, so giving an extra 3 days, and thus 9 months and 9 days.
tidsskriftet.no/en/2012/01/how-long-does-pregnancy-last
So, 9 months and 9 days from last period. Add 5 days for sperm and you get 9 months and 14 days...
That would put the due date easily to the day, and certainly in the same week of the 9th month after the actual sex act and even ovulation.
So, before you call pps stupid, maybe check the actual data.