In what way are they different? Minutes, or hours? Whole hours?

Are the messages coming to you from a different time zone?

It may be whole hours, it seems close enough.

The mail notifying of your reply was timed at 15/03/2015 15:23 which is near 9 hours earlier than my time.

It happens with mail arriving from my time zone too.

Thank you ori

have you checked to see your computer is set to the correct timezone? Not the correct time, the correct timezone.

See http://kb.mozillazine.org/Time_and_time_zone_settings for more details. If you set the local time on your system but are on the "wrong" time zone, all messages (sent or received) will have an offset as noticed in your case.

Correcting the time zone fixed the discrepency.

Thank you to all responders

ori