Doesn’t Understand The “Or” Part

(My husband and I are at the grocery store, picking up a few things. When it’s time to pay, we choose the express checkout line.)

Cashier: “Excuse me; this line is for fifteen items or less.”

Me: “Yes, I know; I have fifteen items.”

Cashier: *gives heavy sigh* “I said; fifteen items or LESS!”

Me: *confused* “Yes, I heard you, and I have exactly fifteen items.”

Cashier: *glaring* “Fifteen items. OR LESS!”

Me: *gives up, goes to another line*