My chiro xray shows lateral pelvic tilt & (as a result) curve in lumbar spine. Doc said the curvature in the spine is NOT scoliosis. How is it not?

Stop seeing chiro. Why are you confused?. This just proves Chiro's can't read X-rays!! Why would you think they could. ? heck, Most MD s can't read X-rays. . If concerned, Only a radiologist should read your X-rays. They spend years learning to read them. Are you having symptoms. ? Or is the chiro just coming up with a reason to recommend a year of monthly 'treatments' Be careful and watch your wallet .
Nobody's perfect . If I take an X-ray on 10 people I'll find something "wrong" in 9 of them. Nobody's spine is perfect. .
Depends on details. The presence of a tilt on an X-ray does not necessarily mean scoliosis. There can be positional curves which show up on imaging that have to do with either posture or muscular tension. Scoliosis represents a curvature to the spine which reflects an abnormality of how the bones interact with each other. This difference can be subtle. A trained radiologist can tell the difference.