There are several possibilities, including:
You had a previous visit with an open balance and your payment was applied to that older balance
Another member of your family, for whom you are the guarantor, had an older open balance and your payment was applied there
Our payments are not usually posted to your account on the same day you pay them. If the date of the statement is close to the date of your payment, it is possible that they crossed in the mail.
Your insurance company required an additional co-pay. Some insurance companies (e.g. GHI, POMCO, Empire Plan) require an additional co-pay for certain diagnostic procedures. This is a requirement put into place by your insurance carrier, not by ENT and Allergy Associates.