Sorry for this silly title. I tried to pinpoint the cause of this behavior but it seems so random.
Usually my shortcut of Annotation Eraser is Shift+Spacebar+8:
But sometimes it becomes Shift+Spacebar+7:
And after I add and erase some annatotions, it becomes Shift+Spacebar+8 again, back and forth, leaving me confused.
I'm pretty sure that I don't change any of preferences.