As Wynn said, it's because the system is set up on a Random Number Generator. Every synth attempt has a chance to succeed, and a chance to fail. If you pay attention and tailor your action to what's happening with the element, use abilities, get the appropriate training, use appropriate gear/tools, and get Guild support, you can increase the chance that you'll succeed on any particular synth.

What sets FFXIV crafting apart from other games -- like WoW -- is that crafting is a mini-game. It's not a "click this button and you''ll automatically make this item" task. If you play the mini-game well, you'll increase the chance that you'll succeed. But there's always a chance that the RNG Gods will frown upon you and you'll fail.