Wrong running distance and pace on Apple Watch 6

Hello . I bought Apple Watch 6 . On the second day of using I was running with Apple Watch 6 and my old Apple Watch 3 (at the same time on different hands) . I was running on the same route , so I know what distance, pace and time should calculate my watches. So on Apple Watch 3 was right distance: 4km and pretty much right pace 8’43’’/km , but on my Apple Watch 6 it was 3,52 and 10’05’’ /km. It is a big difference. Can you please tell me how it can be ?