During reporting, Mandrill strips the query string from your links by default. This is normally what you want, as query strings should usually just be metadata and shouldn't be counted towards the uniqueness of a URL. However, some CMSes may not act this way, so the option to disable this behavior is included.
For example, the following 3 links:
http://example.com/profile http://example.com/profile?option_a=123&option_b=456 http://example.com/profile?option_b=456&option_a=123
Would all be reported as the same link:
If this option is set to false, however, they would be tracked as 3 entirely separate links.
You can control whether or not to strip query strings from URLs using the
url_strip_qs parameter (API),
X-MC-URLStripQS (SMTP), or globally for your account on the Sending Options page.