Okay, suppose it is so. Can you suggest how the efficiency might have been determined? Was it measured by running a single cart down one side and up the other? Is it independent of total path length? (if you change the initial height h1, does it affect the result?) Does it depend upon the slopes of either side? Normal force (hence friction) depends upon angle, so can "efficiency" be independent of slopes?
Perhaps the OP has been given a particular working definition of efficiency that can be applied here?