It doesn't stutter because the renderingtime alters, it's because the GPU doesn't stop drawing and only updates the buffer with the images from the backbuffer when the screen is ready. Which means that frames are grabbed 2 or 3 times (partially fixed with tripple buffering). Please, just read how it works. Then you also understand the implications a static refreshrate causes, and also how a dynamic refreshrate would solve allot of them (and also not use the word "lag" in the same scentence as g-sync). Right now you are just randomly slapping stuff together in an incoherent mess.
Here's a nice read, in layman's terms.
http://hardforum.com/showthread.php?t=928593