Many villager have same job but there is only one block of work

Many villager may have a job but there only one block of work, if the block was removed only one villager lost his job but the others still have a job.

In the screen I have place a lectern and break it but still have librairian also I may have only two farmers because I have composter but I removed them and I still have more than two, I have trade with two farmer too, so I may have only two farmers.

Thank you in advance.