On a map of the moon's gravity field, a mascon appears in a target
pattern. The bulls-eye has a gravity surplus. It is surrounded by a
ring with a gravity deficit. A ring with a gravity surplus surrounds
the bulls-eye and the inner ring. This pattern arises as a natural
consequence of crater excavation, collapse and cooling following an
impact. The increase in density and gravitational pull at a mascon's
bulls-eye is caused by lunar material melted from the heat of a long-
ago asteroid impact.

http://www.jpl.nasa.gov/news/news.ph...lease_2013-184

The hypothesis seems to be an exercise in pulling our proverbials.