For most patents (at least the ones I looked at) there is no more than 1 year difference between "Priority Date" and "Filing Date": often exactly 1 year, and if not, just under 1 year.
This must be due to the fact that one has 1 year to do the actual filing after the provisional one.
However, rarely, the delay between "Priority Date" and "Filing Date" is significantly more than 1 year.
How is this legally possible?