I've been counting steps for about 3 or 4 years. The only time I had a problem was day during that time when my watch reset itself. That was once in more than 1,000 days.
Keep in mind that there are multiple phases to the phone sync. The first is the phone app connection to the device. The second is sending it to Web Connect. It's possible that the device isn't connected so the app doesn't have the latest values. It's possible that the app shows the latest values from the watch but hasn't sent it yet to the central Connect database on the web. A sync should fix that.