Thank you very much for the additional information . Again I am so sorry this happened.
Have I seen anesthetic death before yes but it is not very common. Any time we put a pet under anesthesia there are risks regardless of the age, and, all we can do in minimize the risks as much as possible. I have seen this happen more in cats then in dogs, however in 25 years I have only seen this happen a handful of times.
In my experience the most common cause of death during anesthesia in a senior cat is undiagnosed heart disease. Rarely is heart disease diagnosed in cats until they have a cardiac event such as a sudden blood clot , heart failure or sudden death. Here is a good article on this
Of course a reaction to the anesthesia is possible as well as human error such as giving to much or improper monitoring. This does not sound like it was anything that was done wrong by your vet in fact by giving less then the normal dose it seems that they were being overly cautious and considering his age.
Unfortunately the only way to know for sure what the actual cause of death was would be to have a full Necropsy done including blood work and tissue sample if nothing is obvious on exam.
I wish I could give a for sure reason but without that necropsy all we can do is speculate.
Here is a great site that is very helpful to some pet owners during this time.Click here
I hope this helps in some small way and again I am very sorry for your loss. Please reply back if you have any additional questions