Here is Back Office User’s Guide description regarding User Agent String is Diff from Previous fact:
Specifies whether the current user agent is different from the user agent of the previous session.
Note that the outcome also depends on whether cookie(deviceToken) is maintained or absent in the subsequent calls. For example see the following cases
When there is cookie
if userAgent is different from the previous transaction, fact outcome becomes TRUE.
If same userAgent is being used, fact outcome becomes FALSE
When cookie is absent, fact outcome will be always FALSE regardless whether userAgent is the same or different from the previous transaction. In this case, the previous device is not identified and a new device is created