Is that true for other brands as well, or just Lapua and their subsidiary brands?
Lapua only, not SK.Lapua and sk.
The lot number-derived figure for Lapua (and any box-printed figures for Eley) are average muzzle velocities, not the velocity of any individual round. Depending on the extreme spread of the lot, which is often higher in lesser varieties, the actual velocity can be significantly higher or lower. Unless it's chronographed, there's no way to know exactly what it is.
Shorter barrels may often produce higher average MVs than longer barrels.
At the same time, two barrels of the same length and from the same manufacturer can produce significantly different average MVs for the same lots of ammo.