Hi Robina,
It says the same in my contract. I think it is a standard thing. I wouldn't worry about it too much as if your practice is anything like most practices all over the country you'll be hard pushed to not have enough work.
At my practice we have 8 nurses and 5 dentists, all the nurses have the same thing in their contracts. I am pretty sure it is just standard.
hope this helps
Allison