Every dojo has its own system and its own way of doing things. I once went to this dojo that didn't even have regularly scheduled belt tests, rather when you thought you were ready for the next belt you would tell your sensai that you wanted to test and he would schedule a test for you. You actually had to be the judge of your own progress in that you had to tell the sensai when you wanted to test if you thought you could pass. It doesn't mean you will pass but as I said, each place has its own ranking methods and testing methods.
And like I said, it's a poor policy to have. The student should be more concerned with knowing the material, being able to understand and perform it, especially under pressure, not scheduling their next belt test when THEY think they're ready. One word comes to mind for schools like that; McDojo.