Tire Inflation: Nitrogen vs Air

Nitrogen - 78.0842%
Oxygen - 20.9463%
water vapor - about 1%
Argon - 0.9342%
Carbon dioxide - 0.0384%
Other - 0.0020%

That's the make up of our atmosphere. Notice it's mostly nitrogen. So in essence you are attempting to displace the other ~22%. If they aren't putting the wheel and tire combo in a completely sealed chamber, pulling a vacuum and filling it with nitrogen, the tires aren't going to be filled completely with nitrogen.