The Date of Registration timestamp in View/Edit Affiliates indicates when the affiliate first registered for an E-junkie account, not when they joined your particular program.
Also, bear in mind you don't really have separate affiliate programs for each product that affiliates could join individually; you actually just have a global affiliate program for your entire account, so once an affiliate joins your program, they can choose which hop links or direct links they wish to promote among all the types of affiliate link you offer.