extras from backend

Hello,
i have a problem with my extras.
I create some extras with “Multiply with No. Items” disabled.
In front end it correctly works, when i add a reservation from the backend the extras are multiplied with the No. Items.
Is there a different way to deal extras in the frontend and in the backend?
How can i solve the problem? Is there a bug?
Thank you very much.
Michela.