Each phase/leg of your supply is monitored separately. The aggregated waveform that is displayed in the Power Meter is what you eyeball when you see, let’s say, that the “AC went on”. Sense’s “eyeballs” are on the phases separately. It has to make assessments regarding single-phase (120V) and dual-phase (208/240V) devices based on the “synchronicity” of those phases. Meanwhile other devices are introducing “noise” on each phase unevenly. Things can go wrong on one phase or the other that can throw things off.
An AC unit would be more prone to these mis-reads than a large resistive (208/240V) load like a hot water tank (4kW+) because the AC load is more noisy in itself so the “tracking” is harder. The load is also likely relatively of the overall load. The smaller the compressor, I suspect, the harder it is for Sense to track. I’ve seen very few errors in Sense’s native detection of my HW tank (208V - 4.2kW) but plenty of errors in my 120V, ~800W air conditioner tracking.