Grip is heavily CNS based. It varies a lot from day to day depending on how much pent up stress you have.

This is something that bothers me about every fitness tracker. They are currently all only about the aerobic system. The musculoskeletal system and central nervous system also have independent stress and adaptation cycles. So your tracker can't tell at all if you are ready to deadlift again, only if you are ready for a jog. Of course they don't tell you that so it is easy to go burn yourself out deadlifting and your tracker won't notice until you have dug a hole so deep your CNS loses control of your resting heart rate.