The curved surface of a clear measuring cup, either glass or plastic, acts as a simple convex lens, very much like the ones in eyeglasses.
The light rays passing through the cup are bent in such a way as to magnify the image from inside the cup.
The result is that looking down into the cup shows a lower content reading than the one obtained from looking at the level sideways from outside.
Over the years, most manufacturers have chosen to print clear cups so the accurate reading is readable from outside when the cup is held at eye level.
This may not be the most convenient for the cook, but it is the most consistent.
There is often a significant variation in the contents that will fit into cheap measuring cups and spoons.
A little home experimentation with an expensive set may be in order to make sure of exact measurements.
