As I understand it, since Burst is privately owned (i.e. stock is *not* traded on the open markets), they don't have to publicly disclose full financial information. I don't know about FastClick for sure, but they are probably privately owned, as well.
Discrepancies are, to some extent, unavoidable, due to different systems (the publisher's server stats, the network's stats, the advertiser's stats...) counting things in different ways, along with caching, etc. I have heard that redirection of defaults from one network to another can tend to result in some of the impressions seeming to disappear in the process.
Is deliberate cheating by networks possible? Sure. And cheating is also constantly attempted *against* ad networks, by webmasters who artificially inflate traffic and clicks.
Burst seems to have an excellent reputation for honesty. Certain other networks (I won't name any names) seem to have inspired some "horror stories" with non-payment, etc. Read boards like this one, to get a general idea of which companies people are using and staying with over time.
