In your case when job B gets submitted, it will get assigned tasks, while job A continues to run with fewer tasks.
Fair scheduling is a method of assigning resources to jobs such that all jobs get, on average, an equal share of resources over time. When there is a single job running, that job uses the entire cluster. When other jobs are submitted, tasks slots that free up are assigned to the new jobs, so that each job gets roughly the same amount of CPU time.
Hope this will help.