The libre measures the glucose from the interstitial fluid, which doesn't map directly to the value measured from blood. Think about coordinates, where X is the value from interstitial fluid and Y is the value from blood. You must draw a line somewhere to map the values correctly and the Libre is calibrated in the factory. For me the Libre reader was just an ok indicator about the trend, but the actual value was sometimes not even near the value shown. It's ok between the normal range 4.0-8.5 mmol/l, but I'd be suspicious about every value outside of that range.
With xDrip you can input values measured from blood and the software will try to estimate as accurate values as possible, meaning that in the best case you can be quite sure about the reading, but always better to check from blood.
Remember, that calibration should happen only when the levels are steady and you should calibrate from the whole range to trust the readings. Anyways at least 2-3 measurements per day from blood is suggested, even with the more expensive CGM's.
With xDrip you can input values measured from blood and the software will try to estimate as accurate values as possible, meaning that in the best case you can be quite sure about the reading, but always better to check from blood.
Remember, that calibration should happen only when the levels are steady and you should calibrate from the whole range to trust the readings. Anyways at least 2-3 measurements per day from blood is suggested, even with the more expensive CGM's.