Sent by ChristinaEditor: Without seeing your dutch oven ourselves, we're guessing that yours is made of uncoated cast iron. The ones you see with white on the inside have been coated with an enamel finish.
Uncoated cast iron needs to be kept seasoned and will react with acidic ingredients like tomatoes, while the ones coated with enamel don't have to have to be seasoned and are non-reactive to all ingredients.
Here's some more information on the two:
Does anyone else have an idea?
(Image: Emma Christensen)