Google Maps are not as accurate as yahoo maps

While working on a project to display local heritage in a map format I discovered to my cost that google maps are not 100 % accurate. For some unexplained reason they position grid coordinates about 100 meters further north than they should. The OSI data that I used had the original 6 inch map and the coordinates. Yahoo maps concurred with the OSI map.
To compare the difference click on the following two links and compare the difference. The pointer (which is the location of a fulacht fiadh) is placed above the road in google maps and below the road in yahoo maps.
Google Maps
Yahoo Maps
This raises a number of issues for my project as google maps has a wealth of embedding and annotation facilities. If the maps are not accurate then creating the maps is more problematic as it require manual intervention and correction,

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