I would say that the positive conductor is grounded against something. To test remove the wires from the cav. You could test each pair. You should get approximate the same on both pairs. Around 6 to 8 ohms. If you get something other than that or a variation between the pairs you probably have an issue that will require more testing. I'd also test between a known ground and each conductor.

Iwent thru this exact dance on a job I didn't do. Squirrels at the wiring and was laying on the recessed lighting can.