Case 580 CK rear differential way overfull why?

Hi all. I just bought a Case 580CK and have a bad oil leak coming from the brake cover area. I checked the rear diff oil and it is several inches above the full line. And appeared to be thinner like hydraulic fluid.

I just need to know how it got there?
Is there a seal between the transmishion and the differential that's bad and letting hydraulic fluid in to the diff?

Or did the previous owner put it there?

Case 580 CK rear differential way overfull why?