Efficiency Curve of Buck Regulator
The non-monotonic behavior appears in curves that have auto-skip mode enabled, so that's probably the culprit. 

The efficiency of any SMPS regulator is zero at very low current, because the circuitry takes some power to operate. 

It also will drop at high currents, as the switch conduction losses and I^2R losses in the inductor increase. 

At a level in the middle, where the designer has optimized things, you'll get a maximum efficiency. For a given circuit or chip that optimum can be moved around a bit (sometimes a lot, if it's just a controller chip) by using different inductors or different MOSFETs (if that's an option).