Theres still a chance you could still not be accepted. You could have gotten poor recommendation letters, they don't think you took the appropriate classes in your undergrad, your grades were not as strong as other applicants, did not have as much experience as other applicants etc.
Theres a lot that goes into getting accepted, its more than just a Prof quickly looking at your grades/resume and saying they will advise you.
With that said, having an adviser ahead of time definitely increases your odds of getting in.