Except, these decisions aren't made solely by those professors. It's a committee and, just like any other committee, there are decisions that are made based not on what is best for one person, but what is perceived to be best for the department. Not to mention, as was my case last year, that admissions holds the purse strings and can deny students that the professors have agreed upon. The committee aspect can promote vote exchanges (e.g. "I voted for your choice last year, you owe me") which, despite you being perfect for the department, means you lose out to someone else for reasons entirely out of your control.
We should always take what professors say in September/October with a big grain of salt. They may tell you what you want to hear with the purest of intentions, but the realities of balancing money and departmental politics may very well tie their hands.