The dealer used a slightly too-strong antifreeze mix.
You can check yours with a $2.99 - $5.99 tester from the local auto parts store. I recommend splurging on the bigger Prestone unit as opposed to the one with the little floating balls.
Of course, make sure the engine is cool when you open the pressure bottle.
With the mix strong enough to make the car run below the N, you might find that it never warms up in the wintertime.
