Differences Between Bing Basemap and Bing Maps Content

[WorcesterGTSS]

I recently added the Bing Maps basemap to our organization's Basemap gallery in Portal using these helpful instructions. Looking at the basemap and comparing it with what you see if you visit the Bing Maps site directly, there are some significant differences, even at the same zoom level. Stylistically they look quite different but more importantly (to me, at least), the features included don't appear to be the same. For example, in the basemap version, many of the buildings that are present on the Bing Maps site are missing. The area I'm looking at is Worcester, Massachusetts.

I'm just curious why these differences are present. Is different data being used for the Bing Maps basemap and if so, why is that?