I have almost done 4,000 miles in my current Jazz. Manual calculation of mpg (I have kept Trip B constant since getting the car) is 44.1 - the computer says 48.4.
So, over 4000 miles, the onboard computer is 10% out almost exactly.
I found this on my previous Jazz, my Civic and a VW Golf - 10% seems to be the discrepancy over a reasonable period with most of these gizmos. It varies from tank to tank but over a period - 10% is the figure.
Yes it would nice if they gave an accurate figure but I can work with 10% - tempted to ditch my manual calculations which I do on Fuelly.com except I'd miss the graphs and pie charts!