"MPG" is a realtime calculation updated every 2 seconds. It is an instantaneous reading of the MPG at that moment in time. Even a slight change in conditions can make it change a great deal. Don't use it for anything except entertainment.
"Current" readings are for the current "trip", which starts and stops at your command. A "trip" might be a tankful, or it might be your commute to work, or it might be a drive across the state. The "current" readings are averages and change much more slowly.
This is all in the users manual. If you did not get one they are available online.
ScanGaugeII User Manual : Linear Logic