It's not unusual if the student has prior martial arts experience. Even in other situations, it may make sense. For example, if the student has natural ability and is training very hard, and the dojo generally attracts a crowd that trains less seriously and/or less often, it doesn't make sense to prevent the former student from getting stuck into more difficult training sooner.
That said, if students think they're finding the current training easy, and they're relying on e.g. strength, natural athleticism, flexibility, speed, and/or other students not hitting them too hard because e.g. they're young or female, then promoting them before they've had the chance to really assimilate the deeper parts of the curriculum: the technical body-mechanical nuances, the sparring experience (especially how to block and dodge effectively against a stronger opponent who's not taking it easy on them), the behavioural expectations in the class (and especially sparring), exam, tournament setting etc., and they get promoted to a level where only some of their better abilities are a match for others at their rank, they're likely to come a cropper.
In my first school, we had too many promising young ladies rapidly promoted to a belt or two below black belt, where the students weren't intimidated by an aggressive lady and had the skill and experience to dominate the fight still in an appropriate way, and that was often a huge shock for these ladies who'd been publicly praised at gradings for their aggression and ability, skipped belts, then found themselves struggling, because really they hadn't had the chance and stimulus to develop their defensive abilities at the same rate.
In most schools, someone skipping a grade or not doesn't make much difference in the long term, so I think it's better to encourage such promising students to focus even harder on perfecting the more basic parts of the curriculum - encourage that kind of mentality, rather than a headlong rush.
Still, you might ask the instructor for advice about why they're giving such promotions and how they've seen them pan out in the context of your style/school... I don't see you've got anything to lose by asking. You can do so in a "I want to learn from you as maybe one day I'll be in your situation" way, rather than as a challenge.