When your valves
aren't lined up properly, each valve combination
gives a different timbre. When the timbre changes, a
musician will compensate with their chops in order
to even out the timbre. By compensating with your
chops, you are working harder than necessary to play
the instrument. When you are working harder than
necessary, the upper register is more difficult.
When your valves are in proper alignment, you don't
have to compensate as much to even out the sound.
The upper register becomes easier and more