I don't use Auto so I'm guessing a bit here but I presume you're talking about the plus/minus scale in the viewfinder. If so, I believe it's there so that you can dial in a certain amount of exposure bias should you wish to slightly override the Auto Mode's setting. To use it you would press the exposure compensation button and rotate either then main or secondary command dial to select the amount of compensation you want.
On my D7000 I've had occasions where the camera fired off two shots when I thought I had it set to single shot mode. On at least a couple of those occasions the culprit was the "too easy to change" drive setting dial. Just bumping it can cause it to change modes which is apparently what happened to me. When I checked, it was in a multiple shot mode. However, I thought that Nikon had fixed that in either the D7100 or the D7200.
--Ron
Could anyone specifically share what steps it takes to put the d7500 into multiple shot mode and then back into single shot mode?