I noticed today when doing my collections each city has 1 item that is more rare of a drop then others. (i.e. pie in org, flowers in TB, poem in UC).
What I happened to notice in UC and TB was the profession trainers ALWAYS gave me these items and on more then one occasion instantly refreshed the heart over their head.
I was able to replicate on my alt, maybe it is intended for them to drop only the rarer item since there is less of them then guards. I do not think the instant/fast repop is intended though.
NPC's I used:
Skinning Trainer in TB
Enchanting Trainer in UC
If repost I am sorry.