The average mpg calculators are almost useless, don't worry about how those read at all really. The only way to be accurate is to either manually calculate, or best of all get an app for your phone. Takes 45 seconds when filling up and you'll get dead accurate results.
My 13 CR-V was off every single tank, sometimes by 4-5 mpg. The instant economy thing is even worse, no one drives exactly the same non stop, there are hills even on the freeway. What good is my "instant" economy when it is going to change in 15 seconds anyway?
After this first tank is through my HR-V I'll start calculating mileage and see how it changes over the first few months during engine break in.