No bonus (red) becomes Bonus (green) After logging off

Sorry in advance for not having screen shots.

Nearly all day long my bonus sits at 50%. Occasionally an S plane will be handled poorly and reach the Red (but won’t leave because it’s been fully serviced).
I’ve found the bonus will remain at 50% until the red plane is cleared for takeoff … so I just won’t send it. However, next time I log on the disgruntled red plane has found contentment in life and does not lower the bonus meter to 48%.
Don’t get me wrong, I like exploiting this … er feature. I just wanted to know if it is intentional or a byproduct of something else.