If you've noticed that a particular item number comes up for every patient, it is possible that your doctor has their Default Item Number set to this number. To check this navigate to File > All User Preferences and open the preferences for the relevant doctor.
Open the General tab and locate Default Item No. on the left hand side.
If there is anything entered in this box it will cause the Select Item Number window to appear after the doctor has closed out of a patients clinical window. If they choose Save on this window it will apply whichever item number has been selected to the patient for invoicing.
If you would still like this window to come up but don't want a particular item number to appear by default, simply enter 0 into the Default Item No. box in the doctors User Preferences.