I've got four items overdue, and another due in an hour. While out and about, I complete one overdue action on my iPhone. Before the hour is up, waiting on a park bench for a friend, I do a mini-review on my WiFi-only iPad, deferring the due date on a different overdue action.
Two hours later, I reach a coffee shop with free WiFi. I open the iPad but do not launch OmniFocus; what number appears on the badge? :-)
True that... local notifications could easily push additions for items that were rolling over at a time... But Things does this on a daily basis and with OmniFocus items can have times so it would be more work.
So it's a choice between "Just no it's not updated when the app is running" or "We'll push some updates and make it closer but it may not be absolutely correct." Probably best to understand the behavior and deal with it... "If you need an absolutely correct number you should just launch the app..." :-)