Since the outside temp was 95° and the inside wouldn't get below 75°, I'm guessing that differential.
The outside-to-inside differential is entirely dependent on how well-insulated the house is. Which is going to be different for every house, and entirely unpredictable when the company installs the air conditioner. So if there really is some kind of "20 degree rule of thumb", it doesn't sound logical that the 20 degrees should measured from those two particular extremes.
The last time I had my AC worked on the guy gave me a range on how much cooler the air on the outlet vent was than the inlet vent. I don't remember what he said, but I want to say when he checked the difference was like 26F after he fixed the AC.
The only way the outside temperature is involved is through the heating of the house. The air that is inside the house should be the air that is getting cooled (obviously there is some mixing of outside air, but it should be negligible). If you have adequate insulation, it should be negligible I guess.
I know my house gets a lot cooler than 20F below ambient. It was 108F this weekend and my house stayed at 78F. Of course our humidity is a lot lower here, but I don't see humidity making that big of a difference in the long term cooling.
Edit: I just realized I have no idea where my thermometer gun is. When I get a new one, I'll check the temp from the inlet vent to the outlet vent to compare them.
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