If flu shot vaccines are created from last year's flu, how can it protect against a different stem of the flu?

There are 3 strains: Every year a bunch of experts meet and decide what strains to put in the flu vaccine for the next season. They usually put in 3 different types, almost always one from the preceding year, and then two others that they decide on based on many factors. There is talk that they may start trying to use 5 strains in the next few years. Anyway, you should get a flu shot every year!