The 8.3 l/100Km is the average value in the OBC and when I have checked it by the fill and measure method you describe above, it's been spot on. The 7 l/100Km is using the needle and I agree it's not that accurate.

Sounds like I am worried about nothing with the sooty exhausts although I am tempted to check that the EGR isn't clogged and stuck open because that will lower the temp in the DPF which doesn't help it to burn off soot accumulation.