Submission time always plays a factor in priority (Deadline is primarily a First-In, First-Out queue), and there isn’t a way to turn that off. If you want jobs of equal priority to share the farm, you have to set Machine Limits for them.
We have been discussing the idea of a round-robin queuing system in Deadline, which would allow jobs of equal priority to dynamically share the farm. There aren’t any firm plans in place yet, but we seem to be getting more requests for this lately.