My guess is that it was the only breaker he had laying around. The really bad part is he tried to cover his tracks by scratching off the numbers. VERY illegal in most states. After talking to my buddy who used to be a master electrician, he also suggested reporting him to the state. People can literally die from installing the wrong breaker or wire.
My original advice of just not using the plug at all still stands. Who knows what else this guy hacked together along the way? If you must charge to get to work, I wouldn't charge more than 20-25amps. You'll need the headroom. If the breaker keeps tripping it will eventually fail and that would lead to nothing good.