It's probably good old "normalization of deviance". Smelling exhaust/fumes is common enough that it would not get reported unless someone has a medical event. And reporting it would require investigation and downtime which management would frown upon.

I usually smell it too. I've noticed its when they switch from the air handler provided by the jetway (big yellow hose) to the plane's onboard system. Perhaps there's some sequencing procedure that the staff is moving through too quickly trying to get the plane out on time, or it's just an acceptable amount of cross-contamination.

I don't have a sensitive sense of smell and I still smell it.