This happened to me when I was switching from one device (vivosmart) to another (vivoactive). The switch happen around 8PM when I already had accumulated close to 10,000 steps on the smart. This was, correctly, synced to connect. Then I switched to the vivoactive as the activity tracker. Soon after it reported 103 steps and synced that to connect.
Ever since when I call up summaries for that day it shows 103 steps in the little circular summary display (on connect, on the vivoactive, and on the iOS app), but when looking at the bar graph for the day, you can clearly see all the (earlier) steps. So, that data is there, but it is not summarized correctly.
When I go to "list view" on connect, I actually see two entries for the same day. It appears that, for the summary, only the most recent line item is loaded, but for the bar graph all data throughout the day is used.
Two solutions:
1) When there are multiple entries for one day, add them to produce a new and correct summary, or
2) When an attempted second summary is uploaded, merge it with the prior one
I feel this can, and should, all be fixed on connect and then, once synced, the device will do the right thing too.