In a round, the delay is under your control. If you start singing the first line when you hear me sing the second one, you're actually starting to sing at (second line ts + $latency). And then I hear it another $latency later. A round could work if your call software compensates for this and delays your singing for me until the next round "entry point".

That is, you hear my "Gently" and join in with "Row," but it's not played back for me until I start singing "Merrily." This could work, but as you can see, it would need some clever delay management.