Match2One measures two types of conversions: 

Post-view conversion means that an internet user has seen your ad, and then converted without ever clicking the ad. 

This is a common behavior and usually means that a user saw an ad, opened up a new browser window, and either googled or went directly to your site to complete the purchase/conversion.
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For post-view conversions, by default, we follow the user's behavior for seven days after they have seen the ad.