maybe a strange question but I have a fleet of about Fokker 70's.
All of them have the standard seating config 70Y, however the
range of them varies quite up to 700NM difference sometimes.
Any ideas why this is?
As wildavidson correctly points out, different configurations of the same aircraft can result in very different capabilities like range and airport access. When getting them second-hand, this can be quite a problem as sometimes you have a route that is within the range of your fleet type, but not your specific models (or those available on the market).
As a general rule in the early days of my airline, if there are 2 planes available with different range, I take the one with the longer range even if I don't need it for my planned routes. It may cost a little more in fuel (due to higher max take off weight) but it may also offer the same or less fuel due to a different engine (lower fuel consumption resulting in longer range or less fuel over the same range). This gives me some flexibility in my growth, as I may wish to open a longer route later but not be able to source a plane with the required range when I need it. Grabbing a 'long legger' when I can gives me room to shuffle my fleet around in future.
As I get more established or start ordering new, I tend to pick aircraft for specific routes OR I just take a hit and grab all the longest ranged model, this simplifies the rescheduling of new aircraft.