The first GRB was only detected in 1967. Assuming we have documented every GRB since (we certainly haven't), that means our observational history only covers 0.5% of that 10,000 year expected frequency of occurence.

Assuming our models are accurate, the odds we were just lucky to see this one in our limited observational history are roughly 0.5%. The odds that our models are underestimating the frequency of these events is quite a bit higher.

Time will tell.