I know this probably has been explained somewhere in the forums, but I wasn’t able to find it. However, it is more a curiosity than a problem.
When I start jack with qjackctl, a field in the setup panel shows the expected latency. If I then start ardour, the latency shown in the menu bar is always half the latency shown in qjackctl. Now, I just wonder why this happens, and what latency should I expect from my card (i.e. who is right?).
BTW, I hope you all had a merry Christmas 
latency is not a well-defined term.
both values displayed have a real-world significant meaning. unfortunately, the world of proprietary software has muddied this by always aiming at the lower of the two values as the one to use in claims about latency. ardour’s value is the (essentially) the interrupt interval for the audio interface (under most conditions), and describes the input latency (how long after a sample arrives at the audio interface will it be before the software can access it). qjackctl is telling you the through latency - how long after a sample arrives at the audio interface will it be before it can be delivered back to the audio interface.
Understood. Thanks Paul for the quick & clear reply.