Regulated vs Unregulated power supplies. The power output varies by the power the amp receives as well. This was the reason some used to use "caps" (like a 1 farad capacitor) to ensure their unregulated power supply amp was getting the same voltage from the car. Loads vary and not all car setups are the same, at all.

If you look at "high end systems", people usually beef up stock electrical to combat this.

I've been out of car audio for a while now to lack of funds to play with. Also, read the post by Tec_ about RMS and realize a lot of companies lie about their products purely to push sales. Hell, long ago, companies used to under rate some amps so that people could compete in lower brackets and dominate their competitors with the advantage. They changed rules to stop this.