Hi Martin, While Michael looks at the calc error, here are a couple tools for you to use in this scenario.

Select your object and hit tab and type BoundingBox. This will reveal the erroneous bounds calculation for the object.

Now select the object, hit tab abd type ShrinkTrimmedSrf.

The readout will now be corrected.

Shrinking the underlying surface can be helpful in some areas but also be undesirable in others, where you may be needing to untrim surfaces and such.

FYI.