I think your issue here is based on timing.
If Parent(Global) report suite is 24 hrs
Child report suite is offset by -2 hrs.
Then the issue is Child report suite has a 24 hrs reference data events but when compared to the Parent it now is 24hrs -2 hrs.
If you did a hourly report lets say 5 days do the numbers more closely match when you sum them by matching offsetting hours? The carry over from time difference I would expect to be caught by next day/previous day in the Global.
Unless you have a metric based on UVs or some special processing rule that could be de-duping this is what I think is tripping you up.
GLTU