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Take a look at this Knowledge Base article, I think it should help answer your questions:
The Bing and Esri basemaps all use WGS84 Web Mercator.
When dealing with several miles of distance, I could believe a projection could cause a significant measurement error, but this error is manifested as a tremendous error (~50%) over a very short distance (120 yds). Even if dealing with english vs. metric, a 50% error wouldn't result (1m vs 1yd), so I am still baffled, but thanks for the input.
Web maps usually use Web Mercator projection, which is generally bad for doing measurements. Take a look at this blog posting, which goes into much greater detail about this. The post has a link to an application that shows the difference when using a Web Mercator projection and ones more suitable for length and area.
The scale distortion for Mercator based on a sphere (which web Mercator is) varies as 1/cos(latitude). The equation when based on an ellipsoid is a little more complicated. So if at 40N, 1.3054.