Warranty. I have had some water in mine there as a result of the door being open during a heavy rain, but dried it right up. I also ran a bead of silicone on that seam to limit water intrusion.
I had the whole door replaced due to not being square and rubbing the first 3 months of ownership.
2014 375 BHFS Eagle Premier
2014 Ram 3500 Longhorn DRW CC
6.7 CTD, Aisin, 4.10's