What exactly is McDonald's referring to on their signs when they say "Over (a certain amount) Served"?

This question has caused more than one argument. The 'certain amount' does not refer to the amount of customers served, sandwiches served, food items served, or even hamburgers served.
The number pertains to the number of beef patties served. A hamburger counts as one patty. A Big Mac counts as two. A quarter-pounder with cheese counts as one. A double cheeseburger counts as two. Got it? The pratice undoubtedly started when McDonald's served nother sandwiches besides (single-patty) hamburgers and cheeseburgers.
http://www.aboutmcdonalds.com/mcd