According to what was said in one of the break-out sessions at last fall's big CO Do in Houston, the mealtimes are stretched by for each time zone change the flight makes. IIRC they add 30 minutes to the schedule for meal purposes for each time zone boundary crossed. So with IAH-EWR (or vice versa) you can add 30 minutes to the flight's times to determine if it falls in a meal period or not.

And yes, there are always oddball flights where this doesn't seem to make sense. I got a cold plate last year on a lunch JFK-IAH flight that left around 12:30 PM. Flights out of EWR and LGA leaving around the same time had a hot lunch with two choices. The timetable and website both said "lunch" not "snack" for F on my flight, but apparently the cold plate was standard on that flight.