The "claimed" width of a boot is taken from one size, usually 26 (Head tends to use 27 so be mindful of that). The total dimension of a shell is reduced in a smaller size and increased in a bigger size, so length, forefoot width, instep height, heel width, etc all change as you get a different size. This is a normal thing that all footwear manufacturers follow, not just ski boot manufacturers.
When you get sized on a Brannock device (the foot-sizer thing), you'll notice that foot width (A, B, C, D, etc) is dependent on length. A foot that 100mm wide in size 10 might be a C width, but a 100mm foot in size 7 might be an E width. From the manufacturer's side, it is therefore important to adjust the width of a shoe/boot per size in order to produce a consistently fitting boot from size to size. If all mold sizes were 100mm wide, then on some sizes they would be super narrow, on some medium, and on others super wide. So the forefoot measurement of the new Atomic Overload, for example, is 101mm in size 26 and then 99mm in size 25, 103mm in 27, and so on.
Because molds are scaled in each size, it is of great importance to find the correct shell size and always check the shell fit. This is part of the reason why boot-fitters always tell you to get a shell fit and not buy boots based on shoe size.