Blogger's Known Issues - Permissions, reporting on a problem, says:
"Some users have reported not being able to add members to their blog despite having < (less than) 100 current members. This is because our invite limit currently includes all invitations, including those expired, deleted, outstanding, or otherwise used, in the past." (Apr.20th, 2010)
Private blog membership invitations have often been subjected to abuse1, and a probable reason why membership levels appear to exceed the 100 member1 limit !
As Nitecruzr1 says;
"Once again, we see that if you are inviting your friends to be Authors of your blog, you must be able to trust your friends. The Blogger Permissions wizard can't verify what Google accounts become Authors, because the account used to authenticate membership is the choice of the new member..." in his post, Blogger Accounts, And Blog Membership Invitations.
1 The Real Blogger Status blog.