I don't understand why the ad is being displayed on the search network and not on google.
NoCarrier, if I were to venture a guess, it would be that the advertiser's budget is not sufficient for them to show for every search - and that when you are not seeing them, their ad is not showing due to the starting and stopping of ad delivery that a 'low' budget will produce, over the course of the 24 hour day.
It is literally not at all unusual to see different ads appear, just hitting the refresh button, as a consequence of the advertisers daily budgets being insufficient for ads to show for every search.
AWA