Speakers are a much more complicated load than the fixed value resistor used on a test bench to determine the power spec rating for an amp. First, speakers typically have a resistance curve that varies widely with frequency. An 8 ohm speaker may vary from below that number to way above it. Second, many speakers also present a capacitive load which can be difficult for some amps. And, the amp also has to deal with the signal it gets back from the speaker. Once a speaker starts moving, particularly at lower frequencies, it has inertia, so continues vibrating momentarily after the input signal stops. This turns the voice coil/magnet assembly into a power generator which can return power back to the amp. How the amp reacts to this signal can be an issue.
So, you can see this issue is more complex than just how many amplifier watts are generated. Some amps handle complex loads better than others. The specs alone rarely cover all the bases.