Citando Jaap Keuter do site Wireshark Q & A:
Delta is the difference between arrival of this packet vs. the arrival of the previous packet. It's all at the network layer and reflects the packet arrival at the capture interface (where it's timestamped).
Difference tries to tell us something about the relationship between packet arrival and RTP timestamps. It's the (absolute) difference of the packet arrival at the capture interface (where it's timestamped) vs. the expected time of arrival of the packet at the capture interface based on the RTP timestamp.
So, Delta is nice from a networking perspective, but for the RTP receiver the Difference is much more relavant since it influences the depth of the jitter buffer is must maintain to provide an uninterrupted playout.
(Resposta completa aqui )