A lattice should be scaled in Object Mode only. That means that its Scale and Dimensions are the same (unless you are using Imperial units…). Applying the Scale of the Lattice makes its Scale (1, 1, 1) while its Dimension don’t change. That would be the same as scaling in Edit Mode and that is what we didn’t want to do with Lattices.
Why it has to be like this I don’t know unfortunately.
