Its honestly a matter of programming. To keep a running record of every player's drops over the last year would amass a ridiculously large database of essentially useless information. If the game had to request that massive amount of information, even just upon first launch, Destiny servers would be down from 9/15 - Christmas. (yes, hyperbole, but you get what I'm saying)... They have mentioned that RNG will be weighted to increase your chances of getting drops that you don't have, but thats only going to take into consideration what you HAVE currently, not what you've HAD ever.