airplane mode just turns off all wireless connections .. it doesn't stop your phone from using data
data IS a wireless connection. The point of airplane mode is supposed to turn all these wireless antennae off so they don't interfere with airline electronics, and thus not use data.
So, I'd be back with thegrtraveler's question -- whiskey tango foxtrot?
airplane mode doesn't stop all data from being used unfortunately .. i know what it's supposed to do but there are many phones built differently .. some allow GPS some don't .. some allow FM receiver to still operate, some don't ..etc etc
to point though as i mentioned, just setting it to airplane mode doesn't always do the trick where data is concerned. it shuts the phone down in some regard but doesn't make it completely data incapable (if that's such a term lol)
although hey .. just at first glance that was an easy one to suggest .. it could be nearly anything but best bet is the phone was still using data despite that which it is very capable of doing .. even on a wireless connection, there is a chance data is still being consumed and you are still being dubbed for it .. is it supposed to be so? no, but is it so? as Forrest says .. sometimes.
I agree that it is counterintuitive (and frustrating), but I have also experienced this. To get a full-stop on data traffic, you need to disable each wireless radio (GPS, mobile network & wifi) individually - or turn off the phone altogether. Airplane mode, for whatever reason, isn't consistent across the board.