Nothing has brought this more to the fore than the following two screenshots:
The tops one is the tracking of pageviews for Adsense (the lower figure in the relevant one) and the bottom one for Google Analytics for exactly the same page (the code has only been placed on one page in both cases) over exactly the same period of time. The discrepancy is +/- 300%!
OK, this is for a web page, but it shows you how inconsistent statistics can be even for an 'accountable' medium like the internet.
For Internet TV delivery the results are not dissimilar. I've run tests for Windows Media logs which show that they are widely out from actual delivery.
But if Google can get away with this, what chance for online video ? It's worse than TV 1.0!