The difference is whether you are only supplying a "minutes" or you are also passing hour/day etc.
See the examples:
Every 15 minutes
add_task(task_id='1235', queue='default', minute=15)Every hour on minute 20 of the hour (i.e. 00:20, 01:20 ...)
add_task(task_id='1235', queue='default', hour=1, minute=20)