"They consumed eleven sandwiches."
Translation:Ellos se comieron once emparedados.
I was wondering the same and found some useful comments on some other similar questions:
"In the case of comerse, as has been noted, it's really just for emphasis. We have this in English, too. "I'm going to sit right down and write myself a letter." "I ate myself a whole bunch of grapes." Etc."
So I guess we're supposed to understand "consumed" to be a more dramatic version of "ate" in English.
"emphatic reflexive verbs Many verbs can be made reflexive for emphasis, giving an effect similar to he went and... or if you please in English. With verbs of eating/drinking, the effect is similar to putting up after the verb: se lo comió = he ate it up; se lo robó = he went and stole it."
Just FYI, the term "emparedado" is hardly ever used, if at all, in Spain. We usually call a sandwich "un bocadillo" if it is made with long loaf bread ("bocadillo de calamares/de tortilla/de jamón/de queso, etc.). If sliced bread is used, which is much less popular, unless you want it grilled, then we call it "un sandwich" (don't grit your teeth at the way it is sometimes pronounced, though!).
There's this other function (among a bit more others) for "se" that one native speaker in another forum called "verb of completion". So this "comerse" (se comieron) means they 'ate up' eleven sandwiches--all of it, and I think that's why Duolingo uses "consumed". In my view, verbs with "se" shouldn't always be assumed as reflexive. They're actually a pronominal verb, and reflexive is just one type of it.
See Joejknowles's post above; he laid out his view on it quite nicely and provided great links.