The checkbox "Encyclopedic articles" in the #advanced-search interface is labeled like it would always select the main (article) namespace. But this is not always true. What is does is activating the users default selection of namespaces, which is stored as a user preference. The user is able to change this preference by using the "Remember selection for future searches" checkbox. For anonymous users and users who never changed it the preference is indeed the main namespace only.
Steps to reproduce:
* Unselect all namespaces.
* Select only two: (Main) and Talk.
* Check the "Remember" checkbox.
* Search.
* Now play around with the "Encyclopedic articles" checkbox. It will now select the main and talk namespace.
Collection of possible solutions:
* Ditch the feature to have a per-user "default" entirely, and stick to a per-wiki default that is the same for all users.
* Let the checkbox always select the main namespace, and ignore the users preference. This way the label is correct. The downside is that the users preference is not accessible any more. It will only be used when entering Special:Search for the first time, but is not available as part of the namespace selection interface any more.
* Make the label of the checkbox dynamically change the moment the interface detects the users "default" is not identical to the wikis default any more, and let it say "My default selection" or similar.
* Change the label to **always** say "My default selection".
* Have two checkboxes: one for the users preference, one for the main namespace only. Hide the first if they are identical anyway (which is the case for all anonymous users as well as users who never changed their preference).