Why don't people like to be corrected?

Side note:  No matter what you do in customer service, you're always wrong and the caller is always right, according to them.

I work for a large medical company providing free medication for patients who qualify for it.  We get tons of calls from patients and doctor's offices all day long.  Typicall when an office will call the person will say "I need to check on a several patients"  or "I have 3 patients I need some updates on."  That's fine because it let's you know how to steer the call.  Today a woman from a doctor's office calls and says, "I need to check on a patient."  I helped her with the patient then gave the standard company closing.  She then says, "wait, I have another one I need for you to check too."  My response to her was simply, "for future reference can you please let us know at the beginning of the call how many patients you're checking on?"  She said "well you didn't ask."  My response was simply, "well, you also didn't tell me either."  She said a" patient, meaning 1.  Nothing in her statement would prompt me to ask how many do you have.  Yes, we've had some offices call and want you to check on 15 or 20 patients in one call and we just can't tie up the phone lines for that many patients at one time.  Of course she thought I was being rude and demanded to speak to someone else.  Which I politely put her back in the queue.  I wasn't as much correcting her as I was advising her to let us know at the beginning of the call how many patients you have.