Probably because she likes you (and your girlfriend) and find the both of you good passengers. And she offered the extra courtesy as you are DM members as well.

Pursers do not need any reason to offer something out of the extraordinary. I was once offered a bottle of Krug, plus a bottle Lynch Bages just because the Purser found out I was from her home town too.

But really, if people reading this thread start getting ideas and start asking for freebies, you will be politely but firmly declined. Best to just wait for what is offered.