It would be nice if you could post a photo of the cars with NY plates lined up together. It sounds like the cars that came from NY may have been selected for extra intensive cleaning, and/or quarantined. I think someone mentioned that in the TPA thread.

Also don't forget that using NY plates as a primary indicator of potential contamination alone is not enough. The car might have started out at LGA in August, and have "lived" in, say, Charlotte for the past 6 months. Likewise, a car might have MN plates, but could have most recently come from EWR last week.