hmmm.ok i will bite...if you are 'coasting' i.e. travelling with the throttle fully closed the engine management will adjust eh mixture to approcximately 1:14.7 (petrol/air). now if the throttle plate is closed you onl;y going to get the air allowed to the engine via the idle control mechanism (i.e the idle air bypass in a carb, or in our case the slightly more advanced idle air controller)

now the vacume in the air intlet well controll the volume of air there to some extend (higher vacume will drag in SLIGHTY more air) and the vacume will dependto SOME extent on revs, but i seriously doubt you coast at 70mph in the same gear as at 30 mph ...so the revs wont be directly linear anyway...

now if you have a manafold vacume guage in your car you would note that the max vacume reached tend to be fairly stable (say in a 940 about 20")..so the air usage will be roughly the same, and if the air usage is the same then the petrol usage is also the same (cos its a proportion)...

Now I think that you will find than in the event of it maxing at 99.9 thats down to th computer choosing to DISPLAY that due to limitation of nn.n characters on the display, but it will be calculating at true values..a common trick in real time embeded systems with limited display characteristics.

All in all i would bet on the computer being pretty close, unless you have a flaky sensor - and my comparrison of fuel bought over miles driven over 5000 miles confirmed this, in fact it was weirdly accurate...