Must have been a crapy amp, or the test was setup incorrectly.
MY QSC PLX 3402 is rated 1100 watts per channel from 20-20k Hz @ .05% THD, and I know without a doubt that it does EVERY bit of 1000+ watts into the actual subs.
You are correct that a speaker is an inductance and a resistance. That is why it is rated as an "impedance". That is why all amps are rated at an impedance rating and over a given frequency response. If you tested your amp at a set frequency, say something like 1 kHz, or you tested it using music, I would agree you might have never seen the full power.