I think it is a way for the companies that make the things (looks like a Yamaha spec to me) to attempt to show the robustness of the amplifier section inside their particular component. It would be more meaningful if they published HOW they got those numbers (if they complied with some industry standard such as IEC or IHF or whatever one is still around).
I can tell you that my Yamaha Receiver has 2 ohm dynamic power specs listed in the brochure, but in the manual, it says 6 ohms is the lowest it is safe to operate. I think they are both hogwash. A speaker rated at 6 ohms may dip down to 2 at certain frequencies, and one rated at 8 ohms may also dip to 4 and rise to 12. The magazine "Audio Ideas Guide" (Andrew Marchall) to which I used to subscribe published impedence curves for all of the speakers it tested. It was amazing to see a speaker rated at 8 ohms was only an "8 ohm" speaker from 400 HZ to 2 KHz, and it had significantly different (higher or lower) impedence at different frequencies. Remeber, the rating on the back of the speaker is not an exact, it's a rough average, combined with what the manufacterer WANTS the speaker to be rated at.
Most receivers (the most common form of amplification - outsells all others) are rated as 6 ohm minimum. If you are Joe the Speaker Maker, and you want to sell lots of speakers, if you made them all 2 ohm, you'd only sell them to the people who could afford to buy the amps to drive them. If you made them all 8 ohms, you'd sell a lot more.
To summarize my long-winded answer, you are correct that it is likely there to indicate how the amp reacts to varying speaker impedence.
