This was the email I got:
Dear riders To meet the high demand for PBP 2019 participation, we had to reorganise the starting groups.You have the opportunity to choose a new starting group, while remaining within the same time limit, by pushing the link
https://inscription.paris-brest-paris.org/ by Friday, February 22.Regards,The PBP organising board
So I guess the option to choose depended on what time slot people had originally chosen. The 18:45 was the earliest slot on the dropdown that had any room left, I'd have liked even earlier but I didn't check my email until later at night... oh well, shouldn't matter in the end.