prognosis\'s

I have wondering lately about how they can give people "prognosis's". I sent my dr a email asking him this ?. I don't understand why they can tell you how long you will be disease free on the number on lymph nodes involved. For instance a person with 3 positive nodes, it could either be in the blood stream or not. Same with a person with 30 positive nodes. It could be in the blood stream or not. My ? was, if there is no evidence of disease at say like 2 years for both people, then whey will the person with 30 pos nodes have such a worse chance of being disease free years out? He told me it was because they don't have anything more to use as a marker say that who will be alive in 5 or 10 years. But then when you look up the stats the person with more nodes with have a greater chance of not beating it. Can anyone else shead light on this?
Oh and another little motto of mine now is:
This may be the last chapter of life,
but it sure isn't the last page!! (i made that up)