When a page is restricted to a group, Confluence does not record who created the restriction. Best explained in the following screenshots:

hellothere is a user, in this example. Notice that the creator of the confluence-users restriction is Anonymous.

Output from DB shows that the creator is indeed, NULL.

Affected versions:

Tested and reproducible on 3.5.13, 4.0.3 and 4.1
Does not occur in 3.4.9, so I believe this is caused by the change of user management framework to Embedded Crowd.