In the OP, the reported distance is about 23% lower than that of his friend. That's a big discrepancy and the suggested changes in the circumference between 2096mm, 2114mm, or 2084mm are very small in comparison (less than a 2% variation). So there must be another reason. Either this cyclometer uses something other than the usual circumference in mm as the distance setting, it's defective, or possibly the magnet is placed a bit too far from the sensor and some of the distance was not recorded.

I'd first check it again over a known distance and verify during the ride that the speed is showing up properly the whole time instead of sometimes dropping to '0' for a little while.