Decomposition tree forcing fields into tooltips instead of Explain By?

[OneWithQuestion]

I'm really enjoying the decomposition tree and was adding a large number of various explain by fields.

However, some of them it forces into the tooltips areas, but I really want them to be explain by fields.

They seem to be fields that are text and that are Y/N values in my data model.  "Employee is Veteran"
However, "Employee Gender" doesn't come back that way, it allows it to be an explain by option.

What determines why some attributes have to be under tooltips while others can be "explain by"?

[jdbuchanan71]

@OneWithQuestion 

Is it possible that your "Employee is Veteran" is a measure?  I tested with both a Y/N column and 0/1 column and was able to add both in the [Explain By] section.