UndoLimit and MenuConText in RichTextBox WPF

I set UndoLimit =10 in Xaml. When i run my app:

In Richtextbox, I input 10 times,

1. if i use Ctrl+Z to undo. I can undo 10 times. 
2. if i open ContextMenu by rightClick, then i use Ctrl+Z to undo. I just undo 9 times. 

Someone tell me why I can undo 9 times after open ContextMenu ?