This is known as aliasing and is a fundamental problem in signal processing. You can improve this a bit by increasing your sample rate, a 4m range is a bit short with a 2m range. Try a 10m range.
Here for example the query executed at 1515722220 only sees the [email protected] and [email protected] samples. That’s an increase of 1 over 2 minutes, which extrapolated over 4 minutes is an increase of 2 – which is as expected.
Any metrics-based monitoring system will have similar artifacts, if you want 100% accuracy you need logs.