I originated from the Pen. I hope the 'differential' was the toll and the surcharge (do they charge that for drop offs?).
Typically the passenger (in a metered taxi) pays the toll by handing some cash to the driver as they approach the toll-booth. (Some taxis may have EZpasses, in which case you would reimburse the driver.)
There is no airport surcharge (currently 50 THB) for drop-offs at the airport. Surprised this hasn't been implemented yet?