The answer to this is very complex and in order to understand it, you need to understand that the seasonal influenza virus is a very fast mutating virus. The CDC, WHO and other organizations try to effectively determine which influenza strain is going to be predominant during the flu season. In the end, it’s a highly educated guess, but a guess none the less. During this recent flu season, the determination was not that good.

/U/FilledESinge provided a link to an abstract for a "Vaccines to prevent influenza in healthy adults..

It's very lengthy, but if you are interested in just the results, you can scroll down to the "Effects of Interventions" and "Discussion sections."

Summarized, the inactivated vaccine is only 16% effective in preventing influenza-like-illness, but is 62% efficacious in preventing confirmed flu when the vaccine matches the predominant circulating strain. Those vaccinated with a vaccine that matches the circulating strain visit the doctor 42% less frequently. The efficacy of the live aerosol vaccine is 53% and the effectiveness is 10%. There is a distinction to be made between efficacy and effectiveness. Efficacy is "the extent to which a drug has the ability to bring about its intended effect under ideal circumstances, such as in a randomised clinical trial.” Effectiveness is "the extent to which a drug achieves its intended effect in the usual clinical setting.”. Most adults receive the injectable vaccine, kids are usually the ones who receive the nasal spray so it’s intuitive that the effectiveness of the spray is lower than the efficacy.

Because the influenza virus mutates so quickly, lumping all the results from all the studies doesn’t really give you an accurate picture as to the efficacy of the influenza vaccine. Because the strain is sometimes incorrectly determined, this would bias the studies towards the vaccine being less-efficacious. What really matters isn’t the influenza vaccines of previous years, it’s that the WHO and CDC determine the correct predominant strain for the upcoming flu season. As you can see from the aforementioned study, when the correct strain is determined, the efficacy is around 62%, not great but it’s not dramatically low, either.