Not a bug, it was simply designed so. The graph does not show the last 28 days (4x7), as you may think, but the last four calendar weeks, starting with Monday. So on Monday, it makes 3x7 + 1 = 22 days, on Tuesday 23 days, etc.
It is four last weeks, including this one (not complete yet), so it is four weeks whether you like it or not. The label tells neither the last 28 days, nor 4 full weeks.
Actually, if it really keeps the Monday, 4 weeks ago as the starting point I would agree that it may be a design. Poor but still. I guess I'll keep an eye on it over the next few days.