What about if the trusses were modified in the attic for ductwork or a plumbing vent ? I have seen quite a few homes with enough butchered framing to make you run for your life. Liek the plumber who hacks through 90% of a few floor joists, the electrican who drills 35 holes in a row in the middle of a joist, or the HVAC installer that hacks out every roof truss framing web in the way of his trunk line...
Odds are that the roof was straight when it was framed up 20 years ago.