SF,
P16 is on the money with his answer.
Gauging from the change in prescription you are likely to be of a tender age as when you get older the eye axis elongates.
Therefore your refractive error gets smaller when you are longsighted or bigger if you are shortsighted. That is until you stop growing so most people's refractive error remains fairly stable after they turn 18'sh.
HTH
FD
PS: What happened to the Sywell thread?