The actual state of charge of a Lithium battery can't be directly measured by any instrument developed to date. The cars use a math model based on the voltage and tracking current in and out to calculate the SoC.
It's always been normal for it to adjust up or down a few miles when the car is parked - as the actual resting voltage of the pack can be measured and used to update the model (when the car is on, power flow to/from the pack causes voltage to bounce around.)
What you're describing isn't normal, though. It sounds like the math model is getting some conflicting inputs, resulting in unpredictable behavior. MY guess is there either a failed sensor or a loose connection or cells going bad, but I'm not nearly enough of an expert to tell you exactly which, and I haven't read of this exact issue before.
It's always been normal for it to adjust up or down a few miles when the car is parked - as the actual resting voltage of the pack can be measured and used to update the model (when the car is on, power flow to/from the pack causes voltage to bounce around.)
What you're describing isn't normal, though. It sounds like the math model is getting some conflicting inputs, resulting in unpredictable behavior. MY guess is there either a failed sensor or a loose connection or cells going bad, but I'm not nearly enough of an expert to tell you exactly which, and I haven't read of this exact issue before.