What difference between PageWidth and Width in PageSheet

[Jerry]

When I set the formula in PageSheet
PageWidth --> 210 mm (A4 Size, OK)
Width --> 25.4 mm (I don't understand why)
Height --> 25.4 mm (the same as Widht ??)
In theory, some value in pagesheet, such as xform or char, does not have the
significance, why the strange value is displayed?