Yes, program material has dynamic peaks. That is why you add in some headroom when you are calculating the amount of power needed.
This Crown page talks about power requirements, dynamics in different program material and a few other things.
Is is worth while noting that good 'ol rock 'n roll requires very little head room, maybe 6 db.
To complicate things more, you have a little power 'in reserve' in your receiver in the form of charged capacitors that can handle very brief peaks in power demand. I have seen some pro amps described as 'more ballsy' because they have larger caps capable of sustaining peaks for longer periods of time.
The short answer is that if you give yourself 12 db of headroom and subtract 5 for room gain and then calculate the power you need in average sized rooms when listeneing at around 10-12 feet, you need a lot less power than you would think
