I’ve heard that in GW1 there was mechanic that “prevents” you from over farming same dungeons by decreasing drop chance if you do them over and over again. ( Thats not official, might be rumor )
I can’t speak much to it in GW2 yet as I’ve only just gotten it and still installing, but in GW1 they did put this in several years ago to combat certain “illegal farming”, but it only really ended up hurting those of us legitimately farming for gear or gold.