Think of it this way.
Lv0 = 1hp
Lv2 = 2hp
Dashing when shooting just does +1 to that, so
Lv0 + Dash = 2hp
Lv2 + Dash = 3hp
If it worked that way—meaning the extra damage was added to *all* of the X-Buster's charged shots—it wouldn't be a problem. Unfortunately, however, this only applies to the Level 0 shot; the others continue to deal the *same* amount of damage.
