The reason boosters are needed after 5 or 6 months is antibody levels apparently drop around that time, booster tops that up roughly speaking. Having a booster dose within that time period does not magically make you more immune.

My son (1, NL) got certain vaccines a few days too early (they should've noticed that when appt was made, and now he has to get the vaccines again, or he is not offficially vaccinated against them. Infuriating but otherwise where do you draw the line with exceptions.

By the way, isn't it there something like the "forgetting curve" for the immune system which could justify administering boosters in specific intervals and unjustify the naïve "better sooner than later"?