insurance payout drama: when the math doesn't add up

Just stumbled across a random fact today that kinda surprised me: apparently, the average insurance claim payout often covers only around 70-80% of the actual repair or replacement cost. I always assumed if you're insured, you're pretty much covered, you know? But seems like there's usually some gap left to fill out-of-pocket. Makes me wonder if people realize this before they file a claim or if it catches them totally off guard. Anyone had experiences with surprise costs after an insurance claim?