Where did you get this info? According to the Help in the Google Maps application, when not using GPS, the blue dot is your estimated location. The light blue circle that surrounds the dark blue dot is the uncertainty zone.

IOW, the blue dot is an estimated location based on the triangulation or whatever algorithm is being used, and the larger light blue circle indicates that you could be anywhere within that zone based on the uncertainty of the estimation. Eg. the 1700 meter, 5000 meter, etc. values that it gives.

In my case, depending on where I am with what little testing I have done, both can be completely wrong, and at other times my location is within their tolerance. Since this is a Beta, errors can be expected.

Edit: My guess is that the help is wrong, and you could be correct in that the blue dot is the cell tower location as defined in a database - and there are errors in the documented locations of these towers. It does seem that the blue dots appear at fixed locations and don't vary other than at these fixed locations, including 2 miles out at sea for one tower near me.