I'm talking about the % of happy. This is the number which dictates the empire-wide effects. At 53 there are no negative effects, whereas at 45 you get the maluses explained in the tooltip (these aren't awful btw, the main one to be wary of is the -15% CS if you are at War). Not until lower happy % do really bad things (barbarian spawns & eventually city revolt) start happening. So although it swung by 100, your Empire is so massive that this is actually only 8%, which is reasonable.
The yield thresholds for unhappiness in each city (when you hover over unhappy face in city view, you see like "x more culture would be -1 unhappy") are based on the median yields of the world. When the median changes (every turn) these thresholds also would change but, to stop crazy volatile numbers (we used to have this), each City only updates the thresholds when certain events happen. If a City grows the median is recalculated. I don't know if there are currently other events that do it (like War), but I don't think so?