Yep, this is why you're supposed to get them every year - it's extremely rare for a new strain of influenza to appear (we're talking like 3-5 in the past century IIRC), but rather than attempt to have everyone keep track of which ones they've gotten the shot for, it's easier to just tell everyone to get the current one.

Because of this scattershot approach it's also not clear how long it actually lasts.

This is also one of the main reasons mRNA is supposed to be such a huge boon to vaccinations: one of the major reasons so few are targeted each year is manufacturing costs.