What's the oddness of statements like "some of them but one failed the exams"?

Let’s say there were 100 students.

If 100 students fail, you would say “All the students failed.”

If 99 students fail, you would say “All but one of the students failed.” You have removed one student from the group “all.”

If 50 students fail, you would say “Some of the students failed.” You divided the students into two groups - some passed and some failed.

If you tried to say “some of the students but one failed,” that would mean you divided the students into two groups, but then you moved one student from one group to the other. This makes no sense. Why would you divide the students into two groups, put one student in the wrong group, and then move that student back to the right group?

It’s sort of like saying “Bob and Susie will be group A, and Jon and Chelsea will be group B. But move Bob from group A to group B.” It doesn’t make sense to divide them up incorrectly and then change it, instead of just dividing them up correctly.