IE8 and the X-UA-Compatible situation | FarukAt.eş
http://farukat.es/journal/2009/05/245-ie8-and-the-x-ua-compatible-situation

It is entirely possible to encounter IE8 sending the IE7 User Agent string to the server while using the IE8 Standards Mode rendering engine; conversely, you can encounter the IE8 User Agent string while rendering in IE7’s engine.