Not sure I've ever seen those specs, but from what I've experienced, it's which option the truck has as to how effective the heater feels.
If you've got a non-AC truck without the HD heater option, it sucks. AC and HD move what feels like twice as much air, and IMO is a requirement if the temperature gets into the 40's. (Fahrenheit) The non-AC setup could be 100,000 BTU's, but the fan isn't enough for the large cab. Your feet would cook but that's all.