Why is that surprising? I was apart of a study at UW-Madison where tested economy at different cadences and rpms 50-70 trumped higher cadenced. Plus, change in elevation may include soft (zero) pedaling which will effect avg cadence.
During the bike, velocity, power, cadence, and HR averaged 35.6 km·h−1, 199 W, 84 rpm, and 155 b·min−1, respectively, with minimal variation except for velocity