CrunchBase does not by any means offer stable and comprehensive picture over the years. It has been maintained with different levels of commitment, especially for entering in old data (since Crunchbase did not exist in 2000).
CrunchBase is a great resource, but doing those kinds of statistics without appropriate research of how consistent is the coverage is at least sloppy if not willful negligence.
That is the reason I look at investments from 2005 since most investments prior to that were not entered.
I don't think you did a good job of disclosing or the problems that might lie in the data.
Since I've been following crunchbase both from the data perspective and from the perspective of how much resources TechCrunch is devoting to it, I can assure you it varies wildly.
Also the editorial policy has changed a lot in that period, in about 2007/2008 they started putting much more emphasis on international start-ups. So there are specific skews that you should be aware of and disclose them in the blog post.
So I think you did a nice job, but conclusions are not to be trusted at tall. Yes it might be the most well-maintained open data out there, but it does not make it in any more useful for this kind of analysis.