Its averaging the area of the whole image around the focus point. That means a smaller change in exposure for some scenes. I tried my R just now because it is laying here. In the P mode with auto ISO, it changes exposure from 1/125 f/2.8 ISO 500 for a light object, and then moving the focus point to a adjacent black object, the exposure becomes 1/125 f/2.8 ISO 2500. The difference is very obvious on the lcd. A change of slightly over 2 stops.
Then, with a slightly different distance, I selected Av with ISO fixed at 400, f/2.8, and the shutter went from 1/100 to 1/25 or 2 stops.
The difference was likely due to me being further away the 2nd time, so there was a different average calculated.
In both cases, the difference was very obvious on the LCD.
My SL2 and my 5D MK IV in live view operate the same way.
It is possible that there is a setting that affects it, or the camera has a problem. I use single point AF and exposure sim on.