My theory, which im not sure is true or not, is that they drop more keys at certain times. I key hunted all day with sorc and found nothing (nihlathak only), then around 7 Pm EST i found 6 d keys in a row...
If i'd be rational I guess you simple had a lucky stroke, however... I experienced the exact same thing as you. The first time I went keyrunning I actually found the whole set (9keys) in about 30 runs. The second time I hunted for keys I got exactly 1 key in about 40-50 runs! (same character). I mean sure you can have some goodluck and badluck but damn those marges are huge. Imo there might be some thruth behind that theory.