Do pickup trucks get better gas mileage going faster than 55?

So i know cars are supposed to get optimal gas mileage at 55 mph. for most commuter cars at least but i have a nissan frontier with an open bed and tailgate up. i've driven this thing for 3 years now and record every fuel up religiously. i've found whenever i get on the freeway and stay above around 70-75 i get around 21 mpg but as soon as i drive 55 for the whole trip i get about 19.5. i'm wondering is it coincidence of my trips follow this trend or is there some truth behind it. i usually leave for these trips at like 4 am so traffic is not an issue its been the same route and i fuel up at the same gas station before hand(costco). all highway. same distance. cruise control.