One reason may be that the other property is a more recent sale (8/13), while your property's last sale date is in 2006.
The assessor may have already updated the tax records using the recent sale info.
There is also a minor adjustment to the land value because their lot is larger.
My expereince is that assessed values are very, very rarely correct for valuing the market price of a home - unless there is a very, very current sale that the value is based on.
I also ran the sales for the last 6 months on Magill, Silver Spruce, and Manor hill, and found the following summary info: Single Family Sold Listings: 10 Avg. Liv.Area SqFt: 3,181.90 Avg. List$: $593,710 Avg. List$/SqFt: $188 Avg. DOM: 88.70 Avg. Sale$: $586,550 Avg. Sale$/SqFt: $186
You can always invite the assessor into your home to get a fist hand update and re-assessment.
I would contact a local real estate professional to help with this answer and to save time and money.
(Not me, I'm on the Cape, but I can help you choose a local pro if you want help doing so.)
(Please note: when you choose an answer as a Best Answer, or at least give a thumbs up, it helps those who answer questions here.)