Why isn't chapter 117 translated to English, but chapter 120 is?

This is my current understanding of what's going on with the manga chapters...

There are multiple different scanlation groups working on Tokyo Ghoul. Some groups got different chapters done before others, instead of trying to communicate or figure out who and when to upload; they're just uploading what they got whenever it's done. Give it time and eventually all the chapters will slowly fill in and there won't be any gaps. It's to my knowledge right now that there are a lot of chapters actually released in Japan right now that have not been translated so they're probably trying right now to catch up as fast as they can because they know with the release of the anime; popularity and want of new chapters will be in high demand. Just give it some time to fill in, as you've probably noticed, 9 days ago when you asked this chapter 117 wasn't up but now it is.