I wear my watch and also carry my phone in my pocket, as with many male users. If I have my watch in airplane mode, my watch will count 15,000 steps for a work day. Due to limited movement at times by carrying things, etc my watch doesnt get every single step. My phone will be in my pocket the whole time at work except for use, and will count 15,600 steps. When I get home, I will connect the watch and phone together and let them sync. After the sync, my steps will show 17,300. I imagine this is a calculation of steps not seen my both devices at the same time, steps counted by the watch but not the phone (phone use) and vise versa, adding steps to both counts.
I've done a standard walk of mine with both the phone and watch on, and then just the watch on while the phone stayed home with a count of steps very similar only differing by 200-300 steps out of 14,000 (7.3 miles [11.7km] walk).
If the synced steps were counted twice shouldn't the amounts above become 30,600, not 17,300? In 2 walks that follow the same route, shouldn't the watch only walk be half the watch and phone walk instead of differing only a few hundred? Just my experience using both devices to count steps throughout a day, at 20,000-30,000 steps a day using each device separately and together.
Seems they thought of this and Samsung Health on the phone dismisses duplicate steps giving a more accurate result.