Finding an experienced, local broker, or agent, to help you learn about the area and properties you are interested in can save you endless confusion, wasted hours, and can definately help you not miss a good purchase.
You might call the old listing agent and see if it is still for sale and what she says is the reason for the difference. Millennium Realty (508) 478-7756 Listing Agent: Judy Leonelli (508) 377-9190
Tell her she "owes me one". ;^]
On the listing above it looks like Redfin is using the assessors information (town record). It appears that the listing broker who put the 3580 says he used the public record, but the sq ft value is different. There is an additional 1120 sq ft in the basement.
All that said how can I help you regarding the property? It did not sell and was on the market for 202 days.