What is the difference between the distance estimated from the activity data and from the generated Walk report?

The distance from the activity screen is calculated by multiplying the step count recorded by your tracker and average stride length of your pet. The average stride length is estimated based on the data you entered when you registered your pet: type, breed, age and weight etc. 

The distance from the Walk report that is generated is the actual distance your pet has travelled. It is measured by the distance between all GPS locations that the tracker calculates. The accuracy of this distance is dependent on how often the location is updated and the quality of GPS and GPRS/LTE signal.