Nearest suitable, is what it says : suitable it does not say available or ASAP.
The airport that ticks most of the boxes that is. That day on the crew decision it was Gerona.
If you run a loft scenario in the north east corner of france, with different crew pairing ( Belgium, swiss, german and french) You will see that depending from their country of origin, the suitable airport is different ( everything else being equal), and guess what ? they get home !! even if it takes a bit longer, their confidence level is high enough in order to balance the decision in favour of the home country.
I do not know that case at all, but if the crew was going regularly there, this would explain the decision to go southbound, when initial destination was more like north...