Influenza is an amazing bug. It is constantly variable. It's full of surprises. That's why we have pandemics two or three times a century. It's constantly changing its genetic makeup and is very adaptive. Every year, you have to have guessed the year before what the most likely strains are. Usually in the vaccine there will be two of the A influenza type and one of the B. This year, for one of those three, there was not as good a match as sometimes. You still have protection against the other ones. You still have partial protection against this one. But it's not 100%.
Even where there is an absolutely good match, there are still surprises. Even mid-year—a few years ago, for example—the virus can change. You get a slightly different virus, so some of us get sick. But having the immunization, if you do get sick, you tend to be less sick, less likely to be in the hospital, and less likely to die, because there's some protection.