The MER (Multiple Ejection Rack) was the end result of a Marine A-4 pilot wanting to make the Skyhawk more than a one-trick pony. His relatively crude creation allowed him to carry several more bombs. However, it came into being after the F-4 outboard pylon location became established and as it turned out, was a bit too close to the landing gear doors. The solution was to angle the pylon outboard. As usual, the Navy and the Air Force, although forced to have a common designation for the Phantom, elected different solutions.
The Air Force pylon was angled outboard starting at the lower wing surface, allowing a slightly smaller angle outboard to provide the same clearance.
* See http://tailspintopics.blogspot.com/2013/06/things-under-wings-f4h-f-4-phantom.html