I disagree, I never think of a slice of bread as ett bröd, that would be en skiva bröd. Ett bröd can on some occasions be a loaf of bread or like a bun of bread. For instance there's a story in the Bible where Jesus feeds the people fem bröd och två fiskar, 'five loaves and two fish' in the English version I found right now.
It seems people are being shown something that is not in the incubator here. What we see in the Incubator is one sentence Bröd to be translated Bread, and one image exercise Bröd, to be translated Bread. We don't have an exercise saying ett bröd. I believe there is a bug here, so if anyone could provide a screenshot, we could report it to the Duo technicians.