Perhaps the current source of fatigue is something we don’t measure, e.g. local muscular fatigue or hormonal fatigue. Having said that, it is important that you also take into account your subjective feeling and draw a conclusion combining both pieces of information (objective and subjective).
This approach will allow you to make the smartest decision for your upcoming training and recovery. Generally it is important to not rely on one single marker for your decision making.
Additionally, what you’re experiencing is related to how sensitive the assessment is to setup and environment, and small variations can affect the results.
To improve consistency, please try the following:
Make sure the chest strap sits flat, snug, and level each time
Fully soak the ECG pads (5–10 seconds, rubbing water in well)
Keep your setup consistent (same time of day, similar conditions)
Lie still in a quiet environment during the assessment (no movement or talking)
It’s also important to take into account how you feel subjectively, and combine that with your results when making decisions. Using both objective data and your own perception will help you make the best choices for your training and recovery.
In general, it’s best not to rely on a single marker alone, but to look at the overall trend and context.