Like it says, it's an instaneous indicator. If you are coasting downhill with your foot off the gas, and it's showing 50mpg. you will keep getting 50mpg until the % of grade changes or you run out of road!
It's strongly advisable to be looking at the road ahead at such times, and not looking at the calculator.
What you see in the average MPG box is simply the average of all those instaneous readings over time.
But the instaneous reading can tell you if you are leaving the transmission in a lower gear too long (manually), or if you accelerate too fast to get up to speed, or too slow for that matter so it's working longer in lower gears. Etc. etc.
During every day commuting, the instantaneous reading is quite worthless for me. However, if I'm cruising on the highway I can use it to regulate my speed and acceleration to maximize my mpg. But, for the most part, it's worthless!
I'm using the indicator to try to make myself drive more fuel efficiently. I don't understand exactly how to read it, but when I "floor" it, it fills itself in. So I try to have smoother starts and fewer irregularities in my speed. I guess I think of it as gallons per mile, even though I don't think that is the exact measure it is showing. :20: