Regarding ‘why the granularity wasn’t linear’, I think you probably have got the answer from my brief reply. Just in case it’s still not clear: the reason is that typically as the water time increases, the demand for accuracy also decreases. For example, if the water time is only 1 minute, then a few seconds of difference would matter a lot. However, if you are watering for an hour, a few seconds of difference typically wouldn’t matter as much. Hope this makes sense.