Looking at the Efficiency Graph on page 43, the specified 97.5% efficiency at (17V/13.6V) is actually peak or maximum efficiency.  It occurs around 5 or 6A.  As current increases (from the peak of 5A, efficiency drops down to 94% @ 30A.
Essentially, the controller is losing 3.5% power as a result of (I'm assuming) copper losses located primarily in the magnetics and some fet on-resistance losses.   This is the best the controller can accomplish (MPPT = 17V).  At MPPT = 85V and 30A, efficiency losses are 8%, or 38W when applying 480W.
Sal