Criminal Sentence 280: Hyphen or Em Dash

From a Web site about foods with too much sugar:

"Granola bars. Often deemed a healthful snack, some are tricky-a 1-ounce serving of a granola bar with oats, fruit, and nuts has 11 grams of added sugar."

This sentence is confusing: "tricky-a" makes no sense. It would make sense, though, to use a double hyphen (which turns into an em dash in Word): "some are tricky--a 1-ounce serving..."