Geobids Visitor Reporting will always differ from Google Analytics unique visitor report
View Through Conversions and Click Through Conversions
With Geobid, it is possible to receive conversion notices even when your campaign is paused. This usually occurs because of 2 reasons: conversions which start as a visitor viewing your ad at one point, but leaving and then converting at a later point in time. A View Through Conversion (VTC) occurs when (A) a person sees your ad, but (B) later comes back to your site of their own accord. If you paused your campaign between point A and B, this will result in a conversion being reported to you. Secondly, a Click Through Conversion (CTC) happen when a person (A) clicks your ad, goes to your site and leaves without converting, and then (B) returns later on their own and converts. If you paused your campaign between point A and point B, this also will result in you receiving a conversion notification.
Geobid tracks conversions by going back 30 days to see if a visitors returned and converted. By going to Manage Conversions, you can see exactly what type, and when, you received a conversion.