The only thing I suggest is run the washing machine and nothing else during the cycle.
Make a note of both readings and check the manufacturer's instructions for correct water usage.
Yes, if you have pipework under floors on the ground floor, especially in concrete floors, but this would show when everything was off and/or full and movement on the water meter.
No problem - let me know what you find.
You could try isolating the washing machine to see if the solenoid a are passing, but if this was the case then the drum would slowly fill.