I need some help

I have a D500 with all EV compensation set to zero, on top of the camera and also in the menu, but when I put my sb900 flash on top and take a picture and review it on the rear screen it always shows -7EV. I have checked the flash and I am shooting in TTL BL FP with the EV on the flash set to zero. So why am I getting the -7 on every shot with the SB900 attached. The camera is set to 250 HSS and works fine, I just do not understand why it always shows -7 with flash on the camera and without the flash it is not there.

I just checked and it does not show the -7EV on any flash mode but TTL BL FP. or TTL FP
The pictures are fine I am just trying to understand why this is showing up.

Wow ... you've already done all the checking I can think of, including testing different modes. My first thought was that the on-camera TTL had flash compensation reduced -7, but that should show across TTL modes on the camera itself. I don't know that the camera's exposure comp would display on the flash body.

I don't have an SB900, but can try the same test with an SB910.