@John4714 It's a common complaint. It's no use to you that the guest claimed to have given 5 stars, as many people would say this when put on the spot, even when it's not true.
One thing many people don't know is that if guests enter only the Overall rating, the individual categories will default to 5 stars. Some cases like yours are explained by that. In others, the guest had some issue that didn't fit into any of the categories (but frustratingly, didn't want to mention to the host). And on very rare occasions, an unintended rating might be tapped in by mistake.
At any rate, let's not lose sight of the fact that in virtually all contexts where ratings exist, 4* out of 5 is considered to be very good. Most guests have no idea that Airbnb presents it to hosts as a failure - and hosts can choose not to let it get under their skin.