HeavyComponent
The app is correct (I presume, haven't tried it). The built in battery meter "jumps" in steps, so presents itself as full between 100% and 80%. When the battery drops below 80% it slices one fifth of the battery meter. At 60% next slice, and so forth. If you install batterygraph you can see these Bars represented.
Therefore the widget is (more) correct when showing 84% while the nokia battery meter shows full.