XBIm - boundingbox wrong dimensions with circular buildingelementproxy

I am trying to get the dimensions of the buildingelementproxy in VB.net

with rectangular form the dimensions are correct but when i have a circular object the values are off by approx. 10mm..... strange!

Sub Main1()

   ' Load the IFC file
   Dim ifcFilePath As String = "C:\IFC\2 sparingen.ifc"

   ' Initialiseer het model
   Using model As IfcStore = IfcStore.Open(ifcFilePath)
       ' Creëer de geometrie context
       Dim context As New Xbim3DModelContext(model)
       context.CreateContext()

       ' Zoek naar alle Ifc-objecten
       Dim buildingElmProxy = model.Instances.OfType(Of IIfcBuildingElementProxy)()

       '***************************
       ' Loop door elke buildingelementproxy
       '***************************
       For Each BEproxy In buildingElmProxy
           ' Haal de geometrie op
           Dim geom = context.ShapeInstances.Where(Function(si) si.IfcProductLabel = BEproxy.EntityLabel)

           ' Bepaal de bounding box
           Dim bbox As XbimRect3D = XbimRect3D.Empty
           For Each shape In geom
               bbox.Union(shape.BoundingBox)
           Next

           ' Bereken de afmetingen
           Dim width As Double = bbox.SizeX
           Dim height As Double = bbox.SizeY
           Dim depth As Double = bbox.SizeZ

           ' Druk de afmetingen af
           MsgBox($"BuildingElementProxy: {BEproxy.Name}")
           MsgBox($"Width: {width}, Height: {height}, Depth: {depth}")
           Console.WriteLine()

       Next

   End Using
End Sub