Some users have noticed that after saving an activity that was tracked using the phone and watch connected, that the distance changes once it's saved. This is a major bummer for those users, but the good news? Our team is grinding away to get this bug fixed as I type this! If you run into this problem let us know, so we can update you when it's fixed.
Important Note: Always make sure "Watch + Phone" is selected before starting a connected activity with the devices.