I think a factor in those numbers you refer to Graham is that before the men's race started reports from the women about the heat reflecting from the ground, especially on the ascents, and the condition of some of them at the finish made many of the men, including me take on more water than normally would be the case. Some were even jumping in the canal to cool down prior to the start.
I do not know what advice regarding hydration was given to the women prior to their 11am start by the organisers as I was not there but before the men's start at 1pm there were repeated announcements to hydrate and to carry water, which is not usual for a 5 mile race.
I think the RO and his team should be applauded for their efforts and for the copious supply of water available both prior to and after the race. I could have done without the jug of electrolyte poured over my head at the end though!