Data sheet reading can be difficult!I would be with you if I could see the values vary within the range of +-0.5. But they don't
There is no variation anywhere near +-0.5 °C.
This is not what the manufacturer is claiming! The manufacturer claims that if you buy a DS18B20 sensor and use it to measure a temperature over a range of -10 to 85C, then the measured temperature will be within 0.5 degrees of the "true" temperature (more or less guaranteed).