I have a forum mate who used to work for Blackstar. The only speaker brand he actually trusts for their ratings are Celestion. Anything else he'd match with an amp rated at half the speaker rating.

A lot of valve amps are rated around the point the THD exceeds 5%, i.e. when they still sound clean, but can put out a lot more power when really driven hard. A 100W Marshall amp can put out between 180-200W if really pushed. Transistor amps normally have far less headroom at this point and put out very little extra power if driven hard.

But as Marcel said, the average power level when playing a guitar is a lot lot less. All those gaps between the notes, plus the level dying away after the initial attack really lower the average energy level. I asked Celestion about the 90W rating of the speaker in my 100W Mesa Nomad and they said it was fine.

But until recently there hasn't been a universal standard for testing and rating speaker wattage e.g. at what frequency or mix of frequencies, how long the test went on for and what (if any) level of 'damage' was acceptable. There is now an AES standard, which does give a figure very close to most normal RMS watts figures, but it is a set test with known program material and a set duration.