A company has a graduate scheme, they might be hiring 4 graduates this year, they might be hiring 40. Only one job advert.

External recruiters might then re-advertise the job with the company name removed, planning to funnel people to the company and collect their 20% commission.

External recruiters with several jobs might merge them into one. $250k job for a senior java developer with 5 years finance experience + $75k job for a junior java developer = advertise $250k job for a java developer.

A company might have a slow, centralised hiring pipeline for some roles. Google has a recruiter check the candidate's resume before putting you into a lengthy 6+ interview gauntlet, but only at the end of it do hiring managers actually check if the resume matches an open job. And if course if it takes 2 months to get through the full pipeline, the jobs open at the end aren't the same as the jobs open at the start.