because the first one reads that for all possible values of x (in the natural numbers), whatever value you take could be either odd or even. obviously true!
the second reads that for all x, it is an even number and for all x (in another case!) it is an odd number. which is quite clearly false!
the difference between the two statements is that the first gives one case being either odd or even (which is true, as any natural number you choose can only be odd or even) where the second statement gives two separate cases each claiming something is false..
do you see?