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Hey, Steeve's not a bot, and I was going to welcome you to the site but then realized you've been here longer than me.
As discussed above, it's certainly possible to get better fuel economy driving hilly terrain, but it depends on the vehicle and it depends on the steepness of the terrain.
If the hill isn't so steep that you have to use brakes to slow down, then it's generally more efficient because the uphill portion is like a "pulse", where higher engine load produces more power for a given quantity of fuel, and the downhill portion is like a "glide", where the vehicle idles (or engine off coasts) while gravity does all the work.
An 18 wheeler is a whole different animal though because it has lower drag compared to the amount of weight. It doesn't take much of a hill for an 18 wheeler to build up too much speed coasting and require brakes to slow down. Likewise, the uphill portion is so burdensome on the engine that it isn't operating at peak efficiency.
The one thing you left out is if you've controlled for speed. Speed is the biggest factor affecting fuel economy, so if you're doing 75 MPH on the flat interstate, but 55 MPH on the hills, the bulk of the lower fuel economy is attributable to the increased speed, not less efficient engine operation.
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