Seriously ?

It records the economy at that instant in time (hence why it is called instantaneous). So if you're not touching the accelerator (slowing down for a junction, for example), you're doing 99mpg. If, on the other hand, you're accelerating away at a brisk pace, then you can easily see 5-10 mpg.

On my Leon the instant goes as high as 199 mpg. Floor it and it goes well below 5 mpg.

In truth I never use it, just a toy. Suppose it could be used to entertain the kids (if I had any).