Are Keyword Traffic Tools Reliable?

There are several sources to help us determine how much traffic a key phrase is getting, but they all seem to have a different opinion. You would think they would at least have a consensus, but that is lacking even within different departments of the same company (Google.com). We can't seem to get a straight answer from paid services from Word Tracker, to free tools like the Google Keyword Tool. Allow me to share with you this frustration:
To the left is a list of keyword tools and search data I used for the keyword phrase: "Online Shopping Carts"

First we have the free tools. The free tools give us some numbers that no one can really follow. Google offers the Adword Tool and the Keyword Tool (beta), but the disparity is rather large (2400 vs. 8200). This is very hard to grasp because if Google doesn't have good data, then we might conclude we are all kind of in the dark. Yahoo's search tool does not fair much better (You need an account for this one but its free). When I did a query for keyword searches per month, Yahoo gave a range of 316-6400, but stated that their data may not be accurate. When given such a large range, how can we get a real idea?

The paid tools are no better. The top two paid keyword search companies are Word Tracker and Keyword Discovery. The results of the these tools are so vastly different, I can't belive anyone would pay for this service. Word Tracker claims that with their data they can help you "find niche opportunities and out smart competitors, " while Keyword Discovery offers a host of credible users with positive feedback. I should mention that I only used the free tools these sites provide on their website, but why would they offer an inaccurate free tool to promote their products?

For kicks, I tried one lessor-known keyword tool called Market Samurai. This is a software program you download which provides, amongst other tools, insight on search volume. The number it gave was also considerably different from the others. (see chart below)

The last Keyword Tool is a free tool that takes the data from a few free keyword research tools and comes up with its own estimate, and that is SeoBook's keyword tool.

There is nothing particularly wrong with most of these tools. (with the exception of Google's conflicting resources) I'm not sure who to trust, but it seems that you have to stick with one source. Whenever I look at several sources it gets too confusing. I stick with one religiously and it works for me because its consistent, even if its not always accurate.