Apple Jacks Marshmallow Mystery

You can eat Halloween candy for dessert, you can eat candy-themed cereal for breakfast, and now your favorite sugary cereals come in a Halloween edition with marshmallows! Looks like on Halloween you get candy first thing and last thing.

I wondered how much sugar the marshmallows added to an already sugary cereal, so I checked labels for a 28 gram serving.

Sugar in a 28 gram serving of Froot Loops:
Froot Loops with Marshmallows: 13 g  (46% sugar)
Froot Loops without Marshmallows: 10 g  (36% sugar)

Then I checked the numbers for a 28 gram serving of Apple Jacks
Apple Jacks with Marshmallows: 12 g  (43% sugar)
Apple Jacks without Marshmallows: 12 g  (43% sugar)

No added sugar in Apple Jacks? Were the marshmallows practically weightless? Were the Apple Jacks so sugary that adding marshmallows didn't change the sugar content? I bought a box and weighed the marshmallows, which I found to be about 14% of each serving. What was going on?

A week later I checked the Apple Jacks labels again. This time, they read:

Apple Jacks with Marshmallows: 12 g
Apple Jacks without Marshmallows: 10 g

Instead of finding a science error, or a math error, I found a labeling error. So it's official--there is way more sugar when you add marshmallows!