Check your Surface Model Settings - make sure that you have Adjust Flat Triangles in TBC set to Yes on the Surface - that is likely where the difference is - this setting forces Triangles up and down the slopes of the stockpile and limits triangles forming between e.g. three points on the same base of stockpile line which will create flat spots in the stockpile in TBC that wont be in SCS900 as that forces this setting automatically.
If you output the TIN model from SCS900 and read that into TBC as well as the data (Points and lines) and then make a second surface from the points and lines you will be able to see where the triangulation is different and that will be where your 4-5% difference is almost certainly. The other place that could be a difference is only how the plane is computed between the base points - and that may be slightly different between the two - but my guess it is the former rather than the latter that is the difference.
If you still arent geting agreement after reading this, then send me the SPJ file and the Report data from SCS900 Volumes and also the Surface Model output from SCS900 and I can take a look at it.
Alan