Discrepancy in Overture conversion tracking

consistently over 30%...

[Algebrator]

For months now, I have had Overture conversion tracking installed. The number of conversions (landings on software download page, after succesful order) that overture reports is much higher than the number of actual orders. At first it was a 50% difference. Then I made a change where instead of directly taking buyers to download page, they go to Thank you page which redirects to download in 5 sec. Thank you page has the conversion code. In this way I hoped to prevent somebody getting to conversion code page twice (i.e. to repeat the download). This improved the accuracy, so now I am off "only" by 30%. Could anybody suggest a reason for this over- reporting?
Thanks

[redzone]

We have found that Overture's conversion tracking is skewed in "their favor" consistently by 20-30%.. They didn't develop the technology in-house. They bought a 3rd party company, and in it's original form, was even worse than the present day skew.

[Algebrator]

Thanks Redzone...it makes me feel a little better (not that misery loves company, but I kept thinking that I was doing something wrong)
Anybody else -- similar experience?

[qwik]

My website uses a form page to collect lead information and then has tracking code on the "Thank You" page following. I find that people tend to hit the back button. I can tell because I end up with a dual submission in the database when they click through again. Squirrels..

-qwik