Reliability of MC43 Clock (and cycle time)

With a MC43 ECU setted at 20 ms by cycle time
A J1939 frame is sending every cycle with in a byte the data of seconds computed as following

With PCAN-View, i save J1939 Frames during one minute, then i keep only J1939 frame with data on flank (seconds)

It's expected to have 1000ms on each flank of seconds of MC43 Time.
In conclusion, the MC43 cycle time is not equal to 20 milliseconds, why ?

In fact, some operations are based on MC43 Time, and at end of day, the MC43 time is ahead of real time of few seconds.

Best Regards