I get that only when I'm zoomed in 110% on my browser. Zooming messes with the logic. I don't think it needs more work, it's just a fun demo.
Not zoomed in or out. 2 * 65 = 129.
Amused I got downvotes for pointing out a bug. :)
Probably because it doesn't happen for others (fine on my machine for example) so they assume you were trying to win clever points by doing something that obviously breaks it (e.g. zoom) and by doing so missing the actual point entirely (that this demo is possible at all).
As a FYI I have my browser set on a small perma-zoom on HN. Otherwise the text is on the wrong side of being too small to read.
So no, I didn't set out to break it to score clever points - it just didn't work for me.
If I type 55555 everything disappears
Found myself reminded of a Chaos Computer Congress presentation.