Mine definitely does go up and down in 0.1mpg increments and I'm pretty sure my Audi was the same.As far as I'm aware, the majority of the displays use rounded whole miles and litres.
Most of the time this works out to display a higher than real figure, but it can work the other way if it falls right.
Also, they may display a decimal point, but I've not seen one yet that will change by 0.1mpg - they seem to have set steps (say, 30.1, then 30.9, 31.6 and so on) and the point at which they flick to the next varies on whether your average is going up or down...
Catch it right and it could be bang on - catch it wrong and it could be 5%+ out either way.
Mine definitely does go up and down in 0.1mpg increments and I'm pretty sure my Audi was the same.
Funnily enough after I filled it and drove home it showed over 38mpg over the 5 miles.