Deleting unued variables

I have 4 variables that aren't used in any of my slides (User Count = 0):

Results.PassPercent

Results.PassPoints

Results.ScorePercent

Results.ScorePoints

I've checked my slides and none of them are set up as quiz slides, I can't see anywhere these variables are used. When I highlight any of them, the delete option grays out. 

I've transported all slides individually to a new project file and the variables didn't follow. They seem to be attached to the root of the file. 

Can anyone help me delete them, or prevent them from popping up in the future?

Thanks!  Lisa