Back when I was thieving I noticed this too. However not everything should be seen from an experience rate point of view in my opinion. Gem stall is the only access to loot that makes best in slot amulets for certain bonuses (Str ammy & Magic ammy) that's why I never minded the xp rate being that low. Besides, doing 1 stall at a time is a waste of time anyways, I'd suggest you to do rounds of 4-5 stalls. You get some herb supplies, crafting supplies (gems & hides from silk stall) and even smithing supplies. But anyways, you do you
