The Wikipedia article says > This is used to lower the pitch of the 1-3 and 1-2-3 valve combinations. In other words, the tuning slides described in that sect

The Wikipedia article says > This is used to lower the pitch of the 1-3 and 1-2-3 valve combinations. In other words, the tuning slides described in that section of the article are solving a different problem (which is described in a bit more detail in the previous section). As a1k0n said, a given valve combination is a harmonic series. If you play a major third on a trumpet without moving your hands, the distance between the notes will be 386 cents, not 400. Of course, a good player will do whatever they need to do to sound in tune with whoever they're playing with. My understanding is that the lips are the main thing used to get harmonics of a particular valve combination in tune with each other.