It seems to me that if the end devices are "Meraki MR" devices, it seems to be always accurately (did not measure actual length but reported length looks reasonable to me). If end devices are PC (mainly Intel NUC), also the case.
However, I am getting wrong figures if the end device are Sonos AMP and Dahua NVR. Eg have a Dahua NVR which is in the same rack connected via a 2m patch cable, but cable test reported it as 19m (1Gfdx and all 4 pairs OK).
Yes, I have had "weird" and inconsistent results as well. I don't consider it much more than a base line test and don't completely trust it. That said, I think it is still a useful starting point, but I would not take the results and tell a cable contractor they need to go fix their cable necessarily without closer inspection.