ModelSim -deposit signal

In my design I have an 8-bit tri-stated bidirectional bus. When using ModelSim, I used "create wave" for this data bus and set it as "-deposit". However, once this wave is created, right-clicking on the created wave reveals that it has instead been set to "-drive", and the option to set it as "-deposit" has been grayed out. Why is that?

Thanks for your help,

Steve