Frank,
This is an annoying bug that's been around since late July. It's random and doesn't affect all sites. When it does affect a site, the button will show the "Login with Facebook" button (or whatever custom text you have set) when a user has not already approved your app. The button will revert to "Log In" if 1) the visitor is logged into Facebook and 2) that user has already authenticated your app.
The bug report for the issue is below. Unfortunately, it doesn't look to have any real priority from Facebook:
developers.facebook.com/bugs/207484259379937
I'd recommend the following two things to do:
1) Use the "Does this repro describe your issue?" link on the bug report. The more people that report this bug 'officially' (using that link), the higher the priority it will get from Facebook.
2) Use the SCLogin module's Image or Text link options for the button. These will load an image from your site for the button instead of the Javascript button. That will work the same, and should fix your issue completely.
Hope that helps!
Alex