So is the "BEST" mpg the best all time or what?

e.g. if you were to cruise home downhill after refuelling, then it'll show 99.9mpg and just stay there until you rest?

I guess what's the "BEST" used for and how is it used compared to the "AVERAGE" one which is one that we've all come to know.

Mods: EV mod, VVT-i emblem, sport pedals, OEM cargo mat, JDM Prius interior footwell lighting, DICE iPod kit, OEM all-weather mats, LED licence plate lights, Phillips Vision Plus