I’ve seen similar slow decrease in graphs. My assumpion was that the Youless firmware is interpolating/extrapolating/averaging to convert water pulse frequency to water flow level. Every pulse represents the same amount of water. Fast pulses mean high water flow, slow pulses mean low water flow. If the firmware does nothing, you’d get pulses in the graph instead of a higher or lower graph.
So my guess is that it does something like this:
When the tap is opened, the fist pulse represents the first water intake, but there’s now way of telling if the water flowed fast or slow. It needs the next pulse for that, but that may take some time. It probably assumes slow (delaying the graph a bit) until the next pulse comes and it can get a sense of the speed the water is flowing from the tap.
So, when the tap is fully open for a while, the firmware is seeing high flow and correctly expecting pulses to come in at the same rate. When you suddenly close the tap, the pulses aren’t coming in as expected. So it lowers the expectation for the next pulse, slowly dropping the graph. In the end the total amount will be fine (because it underestimated the start and overestimated at the end).
To put it another way (wish I thought of that first ) It is spreading the pulses over time, based on the frequency of the pulses. At the end the freqency is hard to guess, because you never now if the next pulse will come or not.