SPR A5M XP Rail: Nonstandard?

Moved the scope from my AR-15 to the XP last night just to see what I would see. The scope is mounted to Burris extra high QD rings, so I expected it to be suboptimal, but I did not expect what I found. The rings lock up tight on the rail on the LMT MRP (receiver and rail all one piece) but they wiggle laterally on the XP. A lot. Adjusting the rings did not help.

These rings are too tall for the XP, and QD is not all that useful, so I'm not greatly concerned that this ring/scope combination does not work, but I am puzzled that the XP's rail does not seem to be standard.

Any thoughts? What rings are you using?