Why the measure volume result is different between V8i version and CE version

Hi Dear all, 

I would like to ask the measurement question that I discover in these two versions. (AECOsim V8i SS6 and OpenBuildings CE 10050049en )

This situation is happened on my manager doing double-check the model volume. 

I was used AECOsim to create the model, then measure the volume. The result is 17146.2835m^3.

However, when my manager used OBD to double-check, the result is different, is 17229.8567m^3.

After that, I have test for a single element. 

the result is also different on the some element.

as you can see in the picture below.

Does anyone have some problem or anyone know what's happened on it?