You create a paradox, it just keeps looping (you go back in time and kill your grandfather -> you don't exist -> you don't build the time machine and go back -> your grandfather lives -> you exist -> you build the time machine etc). But every time around the loop there is a certain amount of actual randomness.
Perhaps that atom decays this time around, or that transistor erroneously conducts due to shot noise.
So all we see is the final "fixed point". The iteration where everything ends up going just right to avoid a paradox - perhaps the gun fired prematurely, or the time machine didn't work, or a passing airplane dropped an engine on you, or...
From the perspective of any time traveler, it ends up being as though the universe is conspiring against you if you try to do anything that would cause a paradox.