Great inquiry, and there is a LOT to say on this.
Long and short, there are different standards that people are held to when it comes to promotions. There are levels to this, you often hear that "every black belt isn't created equal", and that is true (for numerous reasons). A person who has started training in BJJ, maybe in his mid/late 30's, and earns a black belt in their mid/late 40's.... will they be the same level black belt as lets say an early 20's Gordon Ryan? Or someone like Buchecha?
The biggest issue in promotions in BJJ today, is that business owners have to decipher between tradition and business. In the sense of lineage and tradition, I think that 75% of practitioners would probably be 1 to 2 belt levels lower than they currently are (I look at my coach, one of Royce's first black belts, came up in the Gracie Academy under Helio and Rorion and Royce/Royler/Renzo/etc.... He told me that in his day, it was 12-14 YEARS to earn a black belt..... Nowadays you see guys earn it in 6). The Gracie's nitpicked EVERY little detail of BJJ, and made sure that you were perfect at it....... The issue that created is that a LOT of guys left the GA, and went off and got black belts from other sources (Marc Laimon was one who famously ran out of there crying). This brings up the business side, as a business owner, you don't want your people getting frustrated and disenfranchised with the art, so often times they are usually promoted to keep them encouraged and coming back. You see this (I especially do) in youth programs.... Sadly this is a "participation award" generation now, where EVERYONE needs to feel special without actually earning it. I've seen/heard/dealt with parents of kids, AND adult practitioners threaten to quit if they aren't promoted (to which, I'm like... BYE MOTHERFUCKER!!).... This is the society we are in nowadays, and if you want to make a living teaching and owning an academy, sometimes you have to ride that line between making money and keeping up with traditions.
There is a quote I heard, from either Lloyd Irvin, or Alan Belcher.... One of those guys who do those "How to run a better gym" seminars..... It was something like of your student base, 90% of your students pay your bills, 10% are your talent..... But of that 10%, 70% may go on to be teachers/coaches, and 30% will be your fighters/elite. As a coach, they will see those teachers/coaches, fighters/elite and hold them to a waaaaaay higher standard than the other 90%, that 10%'s promotions will be less frequent, they'll be highly critiqued, their knowledge and understanding will be called into question daily.... It will test them, it will break them, it will make them want to quit..... But if they continue to show, continue to train, continue to be loyal, at the end of the day they'll be better for it. Even if the students in the 90% get promoted before them, or are similar belt rankings, the truth is EVERYONE knows.... everyone knows that the one guy who shows up maybe once a week for 4 years, vs that 10% elite who trains 1 to 2 times a day, 7 days a week, for 2 years..... everyone knows that the levels are different (and like Royce said, "A belt only covers 2 inches of your ass, you better cover the rest", so you're only as good as you can back up).
In regards to skills and knowledge, there are guys who have ALL the knowledge, Know ALL the techniques, but may not be able to physically demonstrate it due to age or other factors (the example I used earlier of a mid/late 40's black belt vs a 23 year old Gordon Ryan). On the flip side, there are guys who may be world beaters as competitors, but don't know all the BJJ techniques, the self defense system, or how to properly stand up in base (nor can they teach it)...... Just because you executed a move..... let's saaaaaay, sit down, play shin on shin, shoot a leg through for an ashi garame/single leg X and turn it into a Honey Hole/411/Saddle finishing with a heel hook..... And you beat everyone with that.... Doesn't make you a legit black belt (especially if you don't know the other stuff like collar chokes or self defense), just means that you are great at that one thing. Your belt should be an indication that you know your rear bear hug defense, as well as you know your lasso sweeps, as well as your hip toss, and well as your no gi leg locks.
Long answer I know, but it basically comes down to this..... many different reasons to how professors promote, but at the end of the day, just focus on your journey and your promotions will come.... If someone who you feel isn't at the level gets promoted to a level before you... no worries... go out there and roll that higher belt (like my coach says, force me to promote you..... show the knowledge and skill set required for you to get to the next level, force your coach's hand... make him promote you based off your work).
Hope my novel is understandable LOL