Because you have no reason to assume that the one with the most impressions is the top performing ads.
Because of the way impression share is distributed (and because we don't know what ad rotation setting OP had on), the top impression ad might only have the most impressions because Google decided to show it more rather than it actually being superior on any actual performance metric.
Because of the way impression share is distributed (and because we don't know what ad rotation setting OP had on), the top impression ad might only have the most impressions because Google decided to show it more rather than it actually being superior on any actual performance metric.