It could have to do with there are published dive tables for nitrox 32 and 36 and the fill station may not have been comfortable mixing 40% for you if you did not have an advance nitrox card, or their gas is already mixed at 32 and 36 and they were not capable of doing a 40% mix. Some gas come pre mixed and they just dispense it, other fill stations mix it themselves depending on their set up.