I'm running 5.1 and have the stock data monitor/usage widget on my homescreen.
My problem is that the quantity displayed on the widget is completely unrelated to (the correct) quantity displayed in the actual data usage portion of the settings menu.
Basically I have gone into a new billing cycle, the settings menu has acknowledged this and so displays 188Mb used whereas the widget displays 1.4Gb used. (For reference, so that you can rule it out, the WLAN usage is on a different schedule and is on 17Gb)
The problem also persists after removing then re placing the widget.