Probably what you see in VOLUME and Volume are different because are referred to different Variables i.e. in Tekla Structures you can export to IFC 3 Volume variables (VOLUME - VOLUME_NET - VOLUME_GROSS) that give 3 different values.
The label that you see in Trimble connet or in the IFC file could be customized by the user, so if the IFC comes to you from a customer you need ask him which variable he have exported
Hope this can help you, otherwise, please add more details to your request
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