The challenge with calculating this number is that we only have a few data points inside a time range, and they tend not to be at the exact start and end of that time range (1 minute here). What do we do about the time between the start of the time range and the first data point, similarly the last data point and the end of the range?
We do a small bit of extrapolation to smooth this out and produce the correct result in aggregate. For very slow moving counters like this it can cause artifacts.