This is right. Temperature differential. Now I will throw a monkey wrench into the whole discussion. I replaced my 84's compressor and hoses out for parts from an unknown newer 90's car that I bought on eBay, replacing all of the O rings with the green ones as I went. I took it to an AC guy who told me I was an idiot and it would never work with r134. AC guys claim that the r134 will leak thru the old hoses and seals and it we know isn't as efficient. So I replaced the usual parts, but left the condenser & compressor, then evacuated and filled with two cans (probably over filled) of RedTek, along with their oil, after doing a lot of research and scratching my head. R12A is MORE efficient. I think the "professionals" are trying to pay for their expensive machines. Mine blows about 40 degrees less than outside temp. I get 50 if it is over 90 outside. It takes a while to cool it off and sitting in traffic doesn't do it, but I am ecstatic. It will get too cold going down the highway. A 32 year old car with 243,000mi going 80 (Montana) on non ethanol premium and getting 26mpg is cool. Very cool.
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Bob