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they are just an aerial photo. the most recent ones in MA where done it 2005 (at least the most recent ones available to the public at Mass GIS). google may have just gotten around to updating them. The previous ones were 2001 and 2003. Google Earth just takes images from GIS services around the world and compiles the aerial photos. The clarity is based on the height of the plane taking the photos (I think its a plane, they may use satellite, depending on the desired clarity). Both the 2001, 2003 and 2005 images are 1:5,000 so the resolution sould be the same. Probably just taken on a less windy day, or a different stage in the tide which is why you can see underwater features better. Land doesn't really change, so the pictures look pretty much the same. You could probably compare shadows of buidings, etc. to verify if the land photos are different.
some of the photos in Europe are crystal clear compared to the US. I think it just depends on how much the government pays for the photos.
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