Pull a spark plug from each and compare.Id imagine you've replaced them in one by now while the other might be running factory plugs. GM changed the recommended plug type and gap around 2008 or so with a tsb.
I always recommend running a few tanks of fuel treatment like amsoil PI or techron once a year and right after buying a vehicle.
Those two combined things jumped my 2001 tahoe about that amount. The plugs were basically completely worn down at 117k.
Depending on how new the 2003 is to you, it might just need some usage and standard maintenence to clean up it's act. Stupid things like the previous owner not changing their oil enough can have an affect.
Beyond that, simple stuff like air filter, tire pressure, tire tread type, and your driving habbits in each will all have an affect.
I'd also recomend tracking your mileage with something like fuelly.com instead of using the computer when comparing it to another vehicle. That computer number is most accurate only to itself.