Hi Owen,

In AC mode, the meter uses zero-crossings to set the boundaries for the RMS calculations (since doing an RMS calculation on an incomplete waveform creates calculation error). If the 5V square wave only oscillates between 0 and 5V without actually crossing 0, I think the RMS calculation window may be off by few samples, which could account for the error you’re seeing.

Does this explanation make sense? Best
~James