The difference is twofold, ease of calculation and market penetration, I guess.
Every mytaxi job, cash or account, gives rise to commission. Commission / 12 * 100 = mytaxi Fares, mytaxi Fares * 1.25 (for arguments sake) = Gross Taxi Fares.
When I was with Blue Cabs most lads switched the radio off periodically to do some street work for cash. Street work isn't so plentiful given mytaxi's market penetration and it is a lot more risky than it used to be.