If I turn on a light bulb while running a 15 amp saw, the breaker is likely to not trip despite pulling more than 15 amps (eg. 15.54 amps is being pulled with a 60 watt bulb).
One of three things is happening is my guess, either:
a) The 15 amp motors and claimed current used is overstated and I'm not really using more than 15 amps or
b) Breakers actually trip beyond their 15 amps rating. Perhaps something like 17 amps of current will actually make it trip?
c) The 15 amp motor w/ light bulb actually WILL trip the breaker, but it's just going to take "a while" for the bimetal in the breaker to heat up enough to trip.