(I’m going to call them Xs because referring to them as saint’s crosses or whatever can be confusing)

because if it were symmetrical, the white X would look like it’s merely a border to the red, which is not supposed to be the case because it’s a separate symbol.

there’s also optical alignment at play. i didn’t find much about this online but as a designer I’ve learned that diagonal lines, if broken by another object, can look weirdly crooked if perfectly aligned, so if you actually slightly misalign them you can make them look more straight. the red X here is broken, but it still looks centered rather than to the side of the white X.