It's just not an exact science. Especially with the variance the Raven algo has.
In the perfect world where everyone has the same hash rate, Same OC settings, luck didn't count and on the same pool. The calc may still be a bit off.
I'm sure they must update their calc with difficulty changes. Does Whattomine show any difference?
You may need to tweak your rig and change pools and\or mine longer to get a result that you are ok with.
My 30+/- MH/s I was getting 120-150 a day when the difficulty was higher on Threeeye(Now Closed)
I had something profound to add at the end but my train of thought was interrupted by my 4yr old trying to sword fight me with a lightsaber.