Ah, FitBit. I'm guessing the device only stores time stamps and not time-and-date stamps. I'm also guessing they haven't worked out daylight saving time, either. Because apparently going to bed before 1am CDT and getting up at 7:30 CST was only 5½ hours. (It was actually 7½.)
This seems related to a problem I saw Wednesday. I got to Atlanta just past midnight and synced my FitBit to my phone—which had already switched to Eastern time. Since the device never got past midnight, it's daily summary value started at the total step count for Tuesday, so my total steps for Wednesday and for the week were overstated by 16,000.
Look, I get that time zones are hard, but they're not that hard. It's also not hard to use date-time stamps with unambiguous values—for example, always using UTC internally and only using local time to display and calculate things that depend on local time.
I've got a support incident open with them. I hope I can help them fix this problem, but it may be a hardware issue. That's disappointing.
Meanwhile, I'll make my first attempt at a workaround on Tuesday morning after I land at Heathrow. (The flight crosses local midnight in Chicago but not London.) I'll sync the device in Central time before changing my computer's time zone to GMT, so that it will have experienced midnight in its local time zone. Then I'll re-sync in GMT and see what happens. At the very least I'll get some data on what is causing this defect.