Yesterday my youthful four-hand duo were sight-reading two of the Variations in G by Mozart, K. 591. They are not expert, in the way that the word is usually defined. But their grasp of the content of what they were reading was right on target. After each variation I asked what was the main point of the variation and their answers were not only accurate but insightful.

At their age I would have been able to play every note up to tempo - I would have scored 100 on a sight-reading test, but I would not have been capable of that other level of precision.

It is like the difference between pronunciation and comprehension.