I once left my biennial review a bit late to book my usual aeros instructor in our Chippy for an hour, so took a PA38 and a flying-school instructor. No aeros of course (him or the PA38), so we did instrument flying. At one point I noticed that despite cruise power set, speed correct, and attitude as it should be, we were descending. The instructor could not explain it.
I had a look outside. Socking great Welsh hill a few miles upwind of us; lee side sink.
There's more to aeroplane performance than feeding numbers from the manual into an i thingy.