The referenceId property of the RevitInstance data.definition` is the speckleId of the definition object.
In the viewer, we are merging this data for ease of navigation and inspection of the scene. The powerbi data connector, however, gives you a flat list of all the objects from that model version.
I could go into the nature of how Speckle stores immutable object version data, but for now, I could refer you to this answer I gave a little while ago:
Power Bilal has a function he has written that may help shortcut this for you. It will depend on you specific needs which approach is best