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On your terms it may be that a much higher percentage of impressions on Google are from actual searches in comparison to Yahoo showing mostly on their 'search network'.
If you consider that some ads on the Yahoo search network often appear as a result of pre-defined searches on very popular sites (even to the extent of big money terms being the default on any page on file sharing sites), it's not difficult to see a very low CTR.
We have had direct experience of this problem on a different network. Very low CTR's is a red flag for your adverts being shown somewhere innapropriate (at best). I'd be very wary.