Maybe it was a typo on the question card?
It stands to reason that the easternmost dessert in the US would be in Maine. Probably an ice cream on the beach somewhere.
I think you folks are over-examining this,. After all, it’s Trivial Pursuit you’re talking about here, not the Encyclopedia Britannica. Or even The Straight Dope. They’re not known for their critical acumen or accuracy. They said on one card that the fictiobnal Otto Titzling* invented the bra, fer cryin’ out loud.
There are all sorts of microclimate/soil areas in the East that produce effects you would not anticipate. The notes above on the Desert of Maine’s origin give one.
One from my childhood is Pine Plains, an area of Jefferson County NY that was effectively prairie (though not called that) dotted by small patches of scrub cedar and pine (not growing to tree size owing to the sandy soil. This was the location for the original WWII-vintage Pine Camp training area of the U.S. Army, commanded by Gen. Hugh Drum, after whom the base was renamed Fort Drum and expanded under Reagan. This little area of effectively prairie is surronded by areas that are naturally northeastern mixed forest, and is less than a day’s Boy Scout hike from an outlier of the Canadian Shield, the Frontenac Arc.