I haven't seen any mentions of a Bing Optimized IE7. A Google search for the above UA only gives 2 results, which makes me think it's a bogus (bot) one ... unless the browser is running in IE7 or IE8 Compatibility View/Browser Mode? Trident/4.0 is specific to IE8.
When the F12 developer tools are used to change the browser mode of Internet Explorer, the version token of the user-agent string is modified to appear so that the browser appears to be an earlier version. This is done to allow browser specific content to be served to Internet Explorer and is usually necessary only when websites have not been updated to reflect current versions of the browser.
When this happens, a Trident token is added to the user-agent string. This token includes a version number that enables you to identify the version of the browser, regardless of the current browser mode.