Why are 2.1A USB chargers 10W not 10.5W?
More than likely, at the full 2.1 amps load current, the output voltage will have dropped slightly below 5 volts to 4.76 volts - still an acceptable voltage for its use.

Note 4.76 volts x 2.1 amps = 9.996 watts

Come to think of it I believe the old standard for TTL power supplies is 5V +/5% = 4.75 volts to 5.25 volts. See here for confirmation. Also this picture: -



Taken from here.