Driving from CA to Idaho and back a couple times a year, I see that scenario often, but it is frequently with two trucks. I will be ready to pass two trucks in the right lane who are just starting to slow as a long up-grade starts, then the second truck will suddenly jump in front of me to pass the slower truck. Then it’s 10 minute slowdown while he BARELY creeps ahead of the “slower” truck in the right lane. It is absolutely infuriating.
Then, when we crest the top of the grade, the guy who made the pass will go barreling downhill at breathtaking speed, far faster than I like doing in my SUV.
Most big trucks have their engines governed to a similar horsepower; hence the similar speeds between them. Going fast downhill is a compromise: saving wear on the brakes and risking a speeding ticket versus safety factors. The ‘asphalt cowboys’, with turned up engines, are actually some of the safer drivers, since they often take more pride in their abilities, and in their rigs. There are exceptions of course.
When being overtaken, I routinely let up, just a little, to allow a truck by that was taking too long to pass me, whether on a hill, or flatland.
I see that as well and hate it, too.