My assumption is that such information is generated by looking at patients who had the operation at least five years ago—say between five and ten years—and seeing how many of them were still alive five years later. That gives a survival rate, but it is the survival rate as of five to ten years ago. Medical technology has been improving in recent decades, which means that the survival rate for someone who has the operation today is probably better, perhaps significantly better, than for someone who had it five years ago.
One way of getting a better estimate would be extrapolation. Calculate the survival rate for patients who had the operation five years ago, six years ago, seven years ago .... fifteen years ago. Assume that whatever rate of improvement you observe continued; fit the data with a straight line or simple curve. See what extrapolating that line tells you about the risk if you have the operation tomorrow.
Can someone better informed than I am about medical statistics say whether I am correct about how they are calculated, or whether something like what I have just described is current practice?