It's fine for Reddit's purposes. Randomly losing 10% of voting interactions wouldn't be a big deal for them. Of course, that wouldn't be ok for many other applications.
They probably figure that with the amount of clicking on threads that people on reddit do that the site will capture almost everything except for that very last page you upvoted on before you closed the tab and went to bed. However, you're probably getting on reddit soon anyway and the votes will be counted then.