The light bar you described seems like it could be using an internal regulator to actually power the lights. It's possible that this has a maximum output power rating regardless of what you provide to the light bar. This regulator will take the 9-32V input and convert it to potentially a 12 / 24V rail that is actually powering the LEDS.
If you could share a link to what LED bar was used that might help us explain what's happening.
Just to clear up your maths and question about Ohm's law, with the provided information you have said that the Light Bar states Max current of 7.5A and an operational voltage range from 9-32V.
The image below shows the equations you should be using to calculate this, to calculate power with the values you have you should do I X V = P. If you want to work out the max current we should do P / V = I. Knowing this we can
Knowing this we can calculate all possible combinations, as shown below,
Assuming the 7.5A limit is used across all voltages you will be current limited at any voltage below 24V meaning the maximum power output would drop as shown.
From my calculations you can see that the Current rating provided lines up perfectly with the 24V @ 7.5A = 180W so it's likely the numbers you are reading are referenced to a 24V supply. This is also quite a common supply rail for LED's so that seems to make sense.