Q: In some of the Tracking Event messages, I noticed different values for weight, volume and/or pieces compared to the root object of the response ?

A: If you see that the value in these fields doesn't match the overall shipment details available at the root response, it most probably means that your shipment has been split between two flights (or more).
In those case, you should see the same event (code, description, eventDate, eventLocation) being duplicated but with different shipment information.