If its alway 1 hour, then maybe something with Daylight Saving Time (DST)? Check wheter is date dependend.
I can check this only on CFG:Time Zone form? If yes - DST not select for all record
You can try checking the user preferences for those user to see if there is any specific country option selected?
Since this is related to Day light saving which might be adding this offset value.
While it is hard to understand.
I created INC with settings +3 GMT (it's on User form)
All display OK
Change TZ to +5 GMT:
Check INC - date/time in it change to +2 hours as need.
Change TZ to +5 too but another record:
And in INC date/time change only on 1 hour forward (not 2 as need).
What could be the problem, where I can search