BoundingBox attributes vs boxextents - unexpected results

Having an issue with a calculated attribute on EPMDocument type to determine material thickness. Using attributes noted in CS324653 and using the supplied code for minimum model extent size  - the returned value  of the new attribute is not correct. 

As an example - when looking at the values in /servlet/odata/v3/CADDocumentMgmt/CADDocuments('{CADDocumentId}')/Representations for a sample part - the value for 'BoundingBox' is:
      "BoundingBox": [
        -0.0533878,
        -0.0673053,
        0,
        0.0860431,
        0.441427,
        0.009525
      ],
This data appears correct and are dimensions measured from the main part planes.

For this example - if Az is BoundingBox[2] and Bz is BoundingBox[5], then `BoundingBox[5] - BoundingBox[2] = 0.009525` which is correct for this part.

However, if I create a new calculated attribute on the CADDocument type (BoxThicknessmin), and set the formula equal to `abs(boxExtents.Az - boxExtents.Bz)`, the value I get for this new attribute is `"BoxThicknessmin": 0.014859520749434692` which is incorrect.

An incorrect valve is shown even if I make a direct attribute relationship without a formula (e.g. simply put BoxExtents.Bz as the formula). 

Does anyone have any suggestions or explanations on why the calculated value has an unexpected result? Are there any server logs that we could lookup to debug this issue?