Are more people tracking how leads originally get to their site, or the pages they convert on? Example:
A lead googles our company, clicks through and visits the site. Then in two days they see our online ad, and fill in our form.
Original source - organic search (captured from original referrer field in Marketo)
How they converted - online ad (captured from URL parameters to hidden fields)
Which are you using to measure the "source" of your leads?