Drawing graphics javascript article

Hi,
I’m reading this article right now:

In the section A simple character animation
there is a block of code:

  if(posX > width/2) {
    newStartPos = -((width/2) + 102);
    posX = Math.ceil(newStartPos / 13) * 13;
    console.log(posX);
  } else {
    posX += 2;
  }

I don’t understard why we need this line:

posX = Math.ceil(newStartPos / 13) * 13;

and we cant just replace it with:

posX = Math.ceil(newStartPos);

It’s written below the code that:
“This has to be a multiple of 13 because otherwise the previous code block won’t work, since posX would never equal a multiple of 13!”

but if we take any whole number and add 2 to it in each iteration it will eventually be a multiple of 13…