You shouldn't compare one student's pace of promotion with another. Yes, instructors have their favorites, who end up receiving extra training opportunities and focus. Teachers are human -- but this is also because the student in question is able to devote themselves more than another, or works harder, or does more...
For example, a student who is still in high school or college has a lot more time to devote to training than a student who is a working adult, with a demanding career and family. It's quite likely that the kid will be promoted more rapidly than the adult; after all, they're not balancing classes with work demands and family obligations. They have the luxury to devote their time to training...
Even if you could devote the same time -- you may simply not be as talented athletically as another student. Or you may not know that the guy who's getting promoted "too fast" has another black belt in a related system...
And, instructors often do select protoges, who receive special attention. This may be because they simply remind the teacher of himself, or because they show some intangible interest or drive.
Don't worry about other people's rankings. Worry about your own training; is it progressing the way you want? Are you learning what you want? Does the training and pace of promotion meet your needs -- or do you feel that you're simply being taken advantage of? These are the things to consider when comparing yourself to another student -- not "he was promoted before me."