It would be great to see the data that you managed to collect on your E53, but it looks like the spreadsheet didn't attach.

I've been doing the same thing though, tracking my fuel consumption since Feb, and I have found that there is some variability in the comparison between calculated and OBC consumption readings that isn't related to the calibration offset - this makes sense to me - things like how far you fill the tank up, how accurate the fuel bowser is, etc, that would play into the accuracy of the calculation.

The goal for me is to get the OBC reading right in the middle of the expected variation. If sometimes the reading is 0.2 L/100km above the true reading, and other times it's below, then that's okay, as long as it's roughly equal. If I can get it to that point, I'll be pretty happy. I expect that when I do, I'll find the same thing that you did, where the long term fuel economy calculation is pretty much spot on.

I realise that I'm being slightly over the top about it (sorry!), but working in instrument calibration has resulted in my being easily bothered by gauges that spit out the wrong numbers...