The theory here about water hammer and calculations not adding up makes sense. I’ve posted a while ago in another thread that I was seeing weird behavior with calculations. I had all zones with the same settings and advanced settings and one zone would be some percentage point(s) different. I too have water hammer on. I don’t recall actually having bad water hammer when this option was off, but in theory I can see the benefit of having another zone opened for a few seconds while the next is starting up.
Previous thread where I discussed the calculations as being off: Evapotranspiration forecasts - #13 by Gerardv514