You have to take the size of the dojo into consideration...
Larger dojos can have classes focused on specific aspects of the art. Perhaps Monday and Tuesday classes are kata, Wednesday and Thursday are sparring, Friday is weapons, and Saturday is general class. If your schedule only allows Monday and Wednesday class, youāre probably going to be missing out on some stuff. You might occasionally do it during the classes you normally attend and/or shift your schedule to get some other class types in every once in a while, but thereās easily room for gaps in your training.
Then thereās black belts who almost solely go to black belt classes. They may go through lower colored belt stuff occasionally, but how often? Testing for say, 4th dan which Iāve heard is a test that everything in the syllabus is expected to be done at a very high level. 4th dan in our organization carries the title sensei, and as such a 4th dan is expected to have the technical knowledge to be able teach and promote. Not everyone can nor should teach, but a 4th dan should easily be able to run a few classes when needed for a CI, and should be able to work with anyone on a one on one basis.
So a 4th dan has a ton of curriculum stuff they need to know inside-out. People usually get into a routine in their own training and for whatever reasons neglect parts of the curriculum. Itās very rare for a CI who has students at a lot of different ranks, but not that rare for someone who doesnāt teach. I could see the student who doesnāt teach having to focus a good amount of attention on the stuff he/she hasnāt been doing for a while when the time comes to test for 4th dan. Especially standardized stuff; we have beginner and intermediate āself defensesā (which are similar to 1 steps) and yakusoku kumite (which are pre-arranged sparring patterns). I could easily see a 3rd dan who hasnāt done them regularly in quite some time butchering them, especially the lowest ones. That wouldnāt be because theyāre difficult, it would be because theyāve forgotten which ones are which, which foot moves where, which hand does which block, etc. To the best of my knowledge those arenāt typically covered in black belt class. If they havenāt been to too many general classes, theyāre going to be rusty.