When you completely deplete the EV range and then call on the system to provide additional power beyond what the engine can provide, it will further drain the battery. While 2 bars is supposedly "11%" per the app, I've seen it as low as 7 or 8 percent and I imagine it could theoretically drain lower than that if someone put it under heavy strain at 2 bars. I don't recall if it showed 1 bar at 7% but it may have.
@KentuckyKen you should try to take it into the hill country at 0 EV and see how low you can get the battery percentage to go.