On our 2.0 install, a search for my surname returns eight results. When I export these contacts (selecting "all", then using the option to select specific fields), I get a CSV with twelve contacts.
I can reproduce this on sandbox, though when I tested it on Sandbox the search results showed twelve individuals, and then exported a CSV with five entries.
Additionally, if I use the "Internal Contact ID" field in the export, I see two columns labelled "Membership ID (match to membership)", and the contents of these columns differ.
I'll post this in a forum thread so I can include the images to demonstrate it.
Please refer to http://forum.civicrm.org/index.php/topic,2526.msg10755.html