While a mileage-based system was preferred, states in the Northeast were allowed to continue using their traditional sequential numbering. That would result in several exits using the same number with an alphabet soup of suffixes. Eventually, the 2009 edition of the Manual required that every state use mileage-based exit numbers. NYSDOT is still converting its exit numbers, for example, gradually doing so corridor-by-corridor. It's just that no agency wants to spend the money to change every single sign.