-ㄴ데 sets something up and 모르다 then undercuts it: it seems that way around me too, but I'm not sure; it is the new song, but people will walk right past without realising. 모르겠다 is the everyday 'I don't know' — a softened present, not a future. One line is a dig rather than an admission: it's obvious, but you don't seem to know it.