Ok here's my thought. Topic vehicle 06 MDX specs the narrower (but slightly longer) 7317 OF. Topic looks to be the 3593a/9688 application. That being the case, I think it's just a build up, combination environmental material and engine crud on the space between the sealing gasket and the filter outer edge. That area is exposed on topic filter.
The 7317 being narrower, less chance to catch the exterior crud outside the sealing gasket which is on the outer edge of the filter.