Are you guys sure you don't have that bass ackwards?? I'm not sure about suunto, but with the Aeris, Oceanic, Genesis, Sherwood, etc... You can do an air dive, then go straight to nitrox. You just can't do Nitrox and go back to air. The reason being, as someone pointed out, the computer only tracks your O2 clock in Nitrox mode. Since you did a nitrox dive, you've been exposed to higher levels of O2 and it won't let you put it back into air mode... only 21% nitrox... that way it keeps counting oxygen.
Then depending on the computer, the above have a feature called FO2 50 which can be toggled off and on. When on, it will default a dive following a nitrox dive to 21% for the O2 for calculating NDL and 50% O2 for PO2 calculations. This is to keep you safe if you forget to reset before the next dive. If you toggle that feature off, it will default to the same mix as previous.