La Defense station is in a weird situation. The RER/SNCF is in zone 3, so is outside the central Paris ticketing area, hence te extra cost. However, the metro part of the station is considered to be in zone 2, because it's the metro. If you want to buy a "ticket t+" carnet, as opposed to the more expensive z3-z1 one, you have to go to the metro part of the station.

Sounds like you had a great trip, though!