Units of measurement are determined by the report's Measure Unitsproperty. It defines the basic measurement unit for all the unit-related properties of the report and its elements (such as location, size, border width, etc.).
It is equal to one hundredth of an inch by default.
If required, you can change the units to a tenth of a millimeter, by setting the Measure Unitsproperty, either using the Property Grid...
... or via the report's Smart Tag.