For whatever reason, Audi are being VERY slow to provide U.S. dealerships with allocations of S5's. For instance, a high volume dealership might only receive an allocation of 3 S5's for a given month. Well, if that dealership has 20 deposits from customers already, that means the top 3 customers on the waiting list would get those next 3 allocated cars.
As for you seeing a reference to 2 months, that might have been a situation where someone cancelled their order and another customer got to take their spot in line (i.e., car might have been going into production when the customer cancelled).