It has to do with the research and tracking patients at certain times after they have been treated for cancer. Some new research is now using 10 years markers instead of 5.
Part of the problem, the longer time goes on, the harder it is to track patients becuase they move away, some cancer patients fail to keep up their suggested check-ups after a certain amount of time and simply, the cancer centers, hospitals and other lose track of the patients as more time goes on. Some people die of other diseases, accidents etc and are usually not counted in the cancer survival statistics.
The other problem is money. The longer a cancer center, research center or hospital try to track a patient, the more it costs. So basically they use baselines given in a group of cancer patients, based on the type of cancer and even the geographical location to determine the percentages of survival at given rates with in 5 years.
They also hire Acutarial Companies which is a research company that does actuarial studies for insurance companies an others. These companies are statistical researchers and use information from many sources to determine survival rates, death rates, disease rates etc etc etc. They use mathamatical formulas based on statistics and probabilities.