Why is this soup sweet?

[spinbaby]

5. There weren't that many carrots

Three small carrots, one medium yellow onion, three or four ribs of celery, one can of tomato paste, one small organic cabbage, navy beans, chicken stock. None of this seems like enough to make it this sweet. The cabbage had been in my fridge for over a month, so I do wonder if it gets sweet in the fridge like potatoes sometimes do.