[Jack-Devel] xruns with 0ms reported delay

I am using a Python API (https://github.com/spatialaudio/jackclient-python) to instantiate JACK clients in independent Python processes. With reasonable audio block lengths (256 to 4096) it works very well to do pretty demanding real-time signal processing tasks (just FYI and to understand the context). The API allows me to detect occurring xruns with a reported signal delay, coming from the JACK library. Sometimes xruns occur and the reported delay is 0ms, while the audio stream clearly experiences a dropout. I've also seen higher delays reported, but right now I cannot tell under what conditions. I just wonder how the delay times are gathered, how reliable or conclusive they are in general and what an xrun "without delay" might mean?!