I don't know what the mechanism is. Here's the symptom on my machine, though. Let's say pin 13 is currently off, I've previously visited http://xxxxxxx/arduino/digital/13/1, and that this URL is Safari's top suggestion for autocomplete as I start typing "http:/xxxxxxx/ard..." What I'm seeing is that as soon as the autocomplete suggests http://xxxxxxx/arduino/digital/13/1, the pin 13 goes to HIGH.
@jessemonroy650 - I think the bookmark idea would work to avoid this (because I wouldn't be typing the URL), though I'm curious if there's a code workaround.
it's possible that the browser is doing a If-Modified-Since request while your are typing. This would be consistent with the behaviour your getting. And since the webserver uhttpd is a minimal server, it might be getting confused with that request, likely because it is ignoring that part of the HTTP header.
If you are truely curious, You can start here. This is documentation I have not released, but it tells you how to get to source trees.
On getting around this "Safari" annoyance, I would suggest a bookmark. The other solution with code is to add a random number to the end of the URL. This is a common cache busting technique, but I am not confident with this idea. The other idea is to clear recent URL history.