We can only go by what the game is telling us and sadly the sample size of 100 for something like this is nowhere near enough. Now is it possible that the values are slightly off, maybe but unlikely the dev team have taken steps to make sure we don't have the crafting nightmare that was XI with it's you must face this direction under a full moon for maximum effect crap. I could postulate that as you craft more and more approaching infinity the percentage chance should go up and eventually reach 90% but as your sample size is limited to 100 or so that makes it less likely that you will have a perfect 90% success rate as each synth is taken individually rather than as a whole set