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When a user signs up for any event, he becomes administrator - the code assgns administrator role to him.
It does the signup_roles module, function signup_roles_signup_sign_up(). And not only administrator role is granted, but more roles (e.g. editor) but not all (e.g. not contributor). I am not sure on what it depends. Even if I'm changing signup's rights for that roles, granted roles are the same.
I cannot reproduce this, it seems to work fine. Did you check the selected roles at admin/config/people/signup?
I was having the same problem, and sure enough, (in the Advanced Settings tab) signups was configured to grant the roles on signup. At admin/config/people/signup.
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