some of the data on the MFD has two values that you can toggle between. MFD shows a "1" or a "2" in the top right like you have found.
When it is in the "1" mode the data is automatically reset when you leave the car for about 30 minutes or so (might even be up to an hour) and then drive it again. When it is set to "2" mode this data remains until YOU reset it by holding down the "OK" button for a few seconds.
This is probably what you have done to accidentally reset it.
In my cars the "2" mode has been left since i bought the cars so that i can see data averages for the cars life etc.
Having said that it would actually be useful to have a third mode so that you could reset this one for long trips etc... but that's just my opinion.