Actually, it's not like that.. You can't have the same current and a lower voltage. It's physically impossible.
7.2V is not enough for a rkcstr driver to regulate the current for a blu ray diode. It needs 2.25-2.5V on top of what the diode needs. And the diode needs up to 5.7V at high currents, and a little less as it heats up.
If you don't give it enough voltage, the current is not what you set it to, as the driver drops out of regulation. And the power drops with the current, which drops with the voltage.
When the driver is in regulation, the voltage on the diode is whatever is required for the set current to flow through it - the driver regulates the current by adjusting the voltage on the diode - it always gives the diode just enough voltage to achieve the desired current, and then it adjusts this voltage, to keep current the same even as the internal resistance of the diode changes..
So if the voltage is too low, the current is too low as well.
I'm using a constant current boost driver. It needs 2.4-5.5V, and boosts the voltage to the exact value required for the current i set to flow through the diode. Since my driver works down to 2.4V, and protected Li-Ions shut down at 2.7V, the current on my diode never changes, untill it suddenly shuts down. The voltage also never changes, unless the driver changes it, to keep current the same as the diode heats up. All drivers do this. It's how they achieve constant current. The diode never sees the voltage going into the driver. It only gets what it needs from the driver.
For any current, there is only one voltage and the other way around (if the resistance is fixed). Diodes don't have a fixed resistance - it changes with heat. But that's why we use constant current drivers. As long as the driver is in regulation, the current will always be the same, and so would the voltage, only that the voltage changes with heat (the driver changes it).
The reason you got lower power with a lower voltage is because your driver was not regulating, and the current was lower, than you thought. With two li-ions, the full voltage is 8.4V, which is enough for a rkcstr and a PHR at the beginning, but as soon as the battery voltage drops to 7.7V, it's suddenly too little. So you may have measured the right current at the start, but by the time you got around to measuring the power, the current was already dropping.
To sum it all up: If your driver is regulating, the current is the only factor (together with heat of course). Voltage doesn't matter here, as long as it is what the driver needs. If it's less, the current is not what you think it is.
The power climbing HemlockMike observed was because he used a constant voltage source. With a constant current, the driver lowers the voltage, as the diode heats up. With constant voltage, the current climbs as the diode heats up (because the resistance decreases).
A constant current driver should not allow this to happen. The only way this could have happened is if it was set higher, than you thought, but didn't have enough voltage on top of what the diode needs, to keep the current constant. Then, as the diode heated up, and needed less and less voltage, the current went up, and closer to what you set it, so you noticed the power climbing.
If you want to make sure the driver is set correctly, you need to give it more than the minimum voltage it needs. To make sure, you would have to give it at least 6V (for the diode) + 2.5V (for the driver) = more than 8.5V.. And the power supply has to be capable of supplying the current without the voltage sagging.
If the power climbs with heat on a constant current driver, then something must be wrong at a certain point. On a constant current driver, the power should drop with heat. That's what keeps a diode safe.
If you were to measure the current WHILE this was going on, you would notice the current climbing. You also need to make sure, the supply voltage is above the minimum UNDER LOAD.
