Quality, heat isolation, also the crossover if done behind the heads may need a flex coupler, mine did or else it cracked. It could also be that the owner ran the exhaust down the passenger side for a bit and then swapped sides somehow, which would introduce problems of its own (clutch slave heating up, fuel lines in close proximity, fuel tank on passenger side...).

-Bill
'87 4Runner w/ '96 5VZ-FE, 'Red Chili II'
'97 Taco XtraCab 3RZ-FE, 'BlackBean'
TLCA # 13257, Rising Sun 4x4 Club Land Use Coordinator
"He who stops being better stops being good." -Oliver Cromwell