A transistor used in emitter follower mode has the "load" between emitter and ground. Therefore "all" of the output voltage appears across this load. There must always be a drive voltage between base and emitter to produce the base-to-emitter drive current. Hence the emitter voltage can never be greater than the base voltage (both relative to ground). If the base voltage is the PWM output of the arduino - a nominal 5 volts, it follows that the emitter voltage can never be greater than the arduino voltage.