By chance?

[arewethereyet]

Bilateral symmetry doesn't ensure equal spacing, but it makes it much more likely because a mutation in one gene can affect both sides of an organism with bilateral symmetry (which is most animals visible to the naked eye).

There is also a problem with YJ's use of the term perfect. None of it is "perfect" symmetry. That is a strawman.

I come up against the concept of perfect symmetry every time I get new glasses.  My eyes are pretty well lined up but apparently my ears aren't quite level.  Drives opticians crazy.