Yep, and my WAG is......

A Win209 or a Cheddite may not leak in the same area. Its also possible the primer pocket in the plug does not cover as much of the primer as say....My Savage or NULA plug covers. The NULA plug for example covers and supports the entire primer.

The more the primer protrudes from the plug, the less support it has. (obviously)

Its also my opinion that the Win209 primer cup is a bit tougher than some others. It may not swell as much. They seem to handle the higher pressures of Savage loads better than some primers.

I would have to agree, Ron appears to be pretty thorough when it comes to ML maintenance. It would be unusual for him to allow his flash channel to get too dirty. That would have been my pick if he was not the OP.