"ces" and "ses" sound exactly the same in French and since the idea of the sentence makes sense, it cannot be marked incorrect. You have the same thing with some singular sentences which sound the same as the plural. Example: "Il parle français." = He speaks French and "Ils parlent français." =They speak French, both sound exactly the same in French so if you translate "He" when Duo means "They" , Duo simply accepts it without comment.