@Piotr73 The "overall" rating is a separate category. What a lower score there usually means is that whatever the guest found to be imperfect was not reflected in any of the subcategories. For example, issues with temperature, noise, or smells. Maybe the guest found the bed uncomfortable, or the WiFi too slow, the kitchen understocked, or another amenity didn't work well for them. Most of the little things that guests care about, perhaps aside from cleanliness and accuracy, have nothing to do with the rated categories.
It probably wasn't a mistake. The guest is automatically prompted to leave supplementary feedback when they choose 4 or fewer stars, but they're not required to. 4 stars is displayed to them as "Good," so in their mind it is not a negative, but they may have felt there was room for improvement. If they didn't care to explain, all you can really do is suck it up and move on.