One reason is probably because the lens itself is of higher grade glass and sharper on its own, when all settings are exactly the same, its probably the grade, somewhat like milk, for example (not the best) fresh milk and rotten milk,chemically they're both the same (as far as I know) but fresh milk is obviously better because well.... it ain't rotten, hence its better while rotten milk,while is still milk, is rotten and sour,and not good for the body, likewise, the 70-200 is an L lens,hence the glass should be better than the 70-300 which i believe is a normal lens