Normally the longer applications are for rolling recruitment drives, ie looking to replace vacancies long term so looking ahead the 5 drivers are due to retire next year plus other natural wastage.
Short applications tend to be looking to fill a more 'urgent' position a lot sooner. With train driver jobs, due to the high volume if applicants, the TOCs tend to get enough applications to work with in the first couple of days so it's down to how picky they want to be.