Ah my bad misread since people had been talking about KC. 
 
In any case, make sure you are certain of the level of each of the mobs you're killing. That's one thing that can account for things. 
 
The other is simply that we're using UIs that show EP bars in flat percentages. You might be getting a flat 2% in FOB but getting 2.5% in Kurns. 
 
The only true way to determine is to take two characters of the same level (and with the same racial EXP penalties and/or bonuses) and kill mobs that you know for a fact are the exact same level as each other for an entire level and see how many kills it takes to go from 0 to 100%. 
		
		
		
		
		
		
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