Power Rating was based off the type of energy source. The original gundams all got a 2, since it was a battery with a much smaller capacity as compared to the newer stuff. In comparison, Saviour's battery allowed it to fly from PLANT -> Earth without recharging, trying to do this with something like Strike would be suicide. Strike did get a 3 in power for reasons stated on the list. The second generation Gundams had a larger battery capacity, but it was still nothing compared to Freedom and Justice, who were both nuclear powered. Gaia/Chaos/Abyss/Saviour/Impulse all use the same battery and recharging system, which is immensely better than the original gundams, as noted. The reason Freedom, Justice, and Providence got a 7 for power is because Strike Freedom and infinite Justice are supposed to have upgraded versions of the same reactor, and Destiny uses an even more powerful energy source. I wanted to rank them all differently based on that, so 7 is the original NJC nuclear reactors, 8 are the improved version, and 9 was set aside specifically for Destiny.
The average rating is just the three ratings (melee/ranged/power) added together and divided by three *waves at 3th grade math class* and I used this to rank them in the post since its a simple way to get an idea of their abilities overall, but that doesnt mean they have the same strengths and weaknesses. You have to look at the other ratings to find that out.