Cadillac XT4 RPMS fluctuate

I like to set my cruise at 74 mph on my Cadillac XT4 and if I slowly increase my speed till I get there the RPMS are just under 2000 but if I pass a car and go back to 74 the RPMS fluctuate to a little over 2000 and will not go under 2000 unless I slow down and gradually speed up ! I asked my dealer and he had no clue …. I understand it has a energy management system with possible cylinder deactivation on the LSY, I need to know exactly what is going on ?