Null/empty property values

I notice that when I'm doing a compare of two forms, if there is a property value that hasn't been set, it shows a little tiny grey circle or maybe a diamond.  If I've visited the property in Forms Builder, but the value is empty, Forms Tool shows a small green square.  Double-clicking for the detail view in the Compare, both look identical; nothing in there.

What is the significance of the difference between the tiny grey circle and the small green square?  I can click the = sign on either side of the compare and get it to switch between grey circle and green square, but there doesn't seem to be anything I can do in Forms Builder to revert back to the grey circle state.

The specific property value I'm looking at is a numeric value property, if that makes any difference.

Thanks,

Rebeccah

Thanks!