Apple has developed a new algorithm that the Apple Watch uses to measure the electrocardiogram (ECG). The corresponding description is provided in the iOS 14.3 and watchOS 7.2 documentation.
Unfortunately, it is not specified exactly what has changed. Only version 2 of the algorithm is indicated . Probably, the readings should be taken now even more accurately.
iOS 14.3 and watchOS 7.2 release on December 14th.