Yahoo has a "search share" measurement, designed to motivate you to bid above the current average, by promising more of a "search share". Google doesn't have that, but I suspect that they also switch out even top ranking positions, just like they randomize some of the ad positioning.
Also, make sure that you don't have any restrictions that might affect those numbers - geographic, time etc.
The above could possible reduce the accuracy slightly - probably not a high percentage.
Additionally, if you're keywords are on broad, they will be including long tail - (or possibly related) searches. You might be able to get rid of the extra keywords through the Google Keyword Report. (Excuse me, the name of the report is slipping my mind.) You might overshoot a little on count.
This is something that you would want to take into consideration depending on how you've built your adwords account. (e.g. Do you have all 3 match types setup for each keyword? If so, you want to add all 3 numbers together, and subtract the Google keyword searches that came up for broad or phrase match.)
Bottom line, the research you get will be much more valuable than wordtracker
