Need assistance to understand Power and Ohm's Law
You're assuming that the current is always 7.5A.  Manufacturers often quote the maximum current draw.  If the light bar contains a switch mode power supply, then the current draw will decrease if you increase the voltage, keeping the wattage approximately constant.
Just because the light bar contains 36 x 5W LEDS, it doesn't mean that each LED is actually being driven at 5W.  Running an LED at less than its rated power makes it easier to keep it cool.  It's also a very good way to fool customers into thinking they are going to get a light that's a lot brighter than it really is.