If I've suckered you into thinking I'm asking that question and you have an answer why then I apologize ahead of time. If, on the other hand, you're reading this thread because you wonder the very same thing I'd like to enlighten you on how an amp works and why some are stable into lower impedances than other comparable wattage amps. I'm also going to address some lies and or half truths I often see that the less informed might take as being true.
First, how do amps function? An amp uses mains voltage/amperage to add gain to an electrical signal from an outside source. An amp is comprised of two parts, the power supply (which is key to what your amp is designed for) and the amplifier section. The power supply takes the 12V DC from your car and uses PWM to turn that power into higher voltage AC. That AC voltage is sent to the amp which modulates that voltage and amperage from the PS (power supply) and sends it out to your speakers. Now, how much impedance the amp is designed to cope with will determine the voltage that the PS supplies to the amp. Amps with .5 ohm stability will require lower voltage at .5 ohm to reach the same wattage as an amp designed for 4 ohms will. Lets do some math. Let's say that we have a 4 ohm sub and we want to put 500W into it. 500W = 44.72136V but on the other hand it's only 11.18034A. The power supply in that amp will need to be designed for running high voltage. If you tried to push that voltage into a lower ohm load you'd end up pulling more amperage than the MOS-FETs in the PS are capable of pushing through the amp as well overloading the output transistors. Let's look at what the same 500W means to a HC amp. That same 500W at .5 ohms drops the voltage down to 15.81139V while the amperage rises nearly threefold to 31.62278A.
This is why that amps designed to run at super low currents are rated low at 4 ohms, the power supply voltage is low since the amp draw goes so high. The reason I bring this up is that I see statements about 1 ohm stable amps being able to push the same wattage into 2 ohm loads and even 4 ohm loads if you just crank the gain on the amp. This just isn't true. An amp that's designed to push 1000W into 1 ohm simply can't push the same voltage as an amp designed to push 1000W into 4 ohms, the PS would self destruct when put under the low impedance load unless it was tightly regulated. An example of an amp with a tightly regulated PS would be the JL SLASH V.2 or the PG Xenon series of amps. Their PS sends a high voltage signal at higher ohms and as the load drops in value the voltage is dropped and the amps increase. The wattage remains constant. An amp with an unregulated PS or loosely regulated PS will increase amperage as the ohms drop until the amp either draws too much current for the PS to handle and it either goes into protect or something pops and the magic blue smoke escapes. It's all based on the working voltage of the PS and the number of components. Increased transistor count equals higher wattage.