The function below is responsible for returning the delay value that is used by the qdisc. If no distribution is specified it is uniform, this is why I didn't see a uniform.dist file in /usr/lib/tc where the other distribution tables are.
mu == latency
sigma == jitter
You can see from the code below that if you don't specify a distribution then it uses the uniform one by default. return ((rnd % (2 * sigma)) + mu) - sigma; in this case rnd is our correlated random number from get_crandom so we take the jitter value sigma and multiply it by 2, then we modulo rnd by this value and add mu our base latency and then subtract sigma to center the jitter