Home assessments in Greenwich (or any other town in the country for that matter) have always been a point of contention. Regardless of where you live always keep in mind that the assessment that the town has placed on your house is one (most likely) person's view of your house at a certain point of time without going inside (most likely), so these numbers are always very rough. If you are interested in contacting the Greenwich Town Assessor or want to look up more information you can find everything at the Greenwich Assessor website.
Please feel free to email or call me at scott@elwell.com or 203.940.0444 if you want to discuss further.