It might help here to say "changes over time, at a rate faster than the production cycle of the vaccine".

In other words you can't usefully produce a vaccine based on the flu seen in the public: by the time the vaccine batch is ready it will be some other strain that is the problem, and the vaccine you made does no good.

In effect vaccine manufacturers have to guess ahead of time what will be needed. Sometimes they guess right and sometimes not. In years they don't you have a big stockpile of the "wrong" vaccine that is "wasted" because it is not needed, and the vaccine you do need isn't available.

To understand why anyone would "waste" huge amounts of money on what might be the wrong drug read up on the Spanish Influenza (which started at Camp Funston in Kansas, not in Spain). Public health officials will go to any length to avoid a repeat of that.