It generally does match (or closely approximate) what it says in Data Usage, however I have noticed that it occasionally resets for reasons I have yet to figure out. I think it might be tied to some reset events (manual/battery pull/battery empty), but this is not consistent at all. If I keep a decent charge in it, or only change the battery before it gets below 5% I do not seem to get this data stat reset.
It might be that you had a similar reset of the data variable in Zooper. I would be curious to see if the gap between Zooper and Android remains consistent or widens.