> Is it not just timing? I thought the effectiveness of the vaccines only lasted around 6 months
No, it is not (just) timing.
"Three doses gives better protection than two doses ever did" (1)
Immune response is very complex (2) and we're not experts; but we should not measure it in just 1 number: that gives the impression that the response is a range from "high alert for this recent threat" through to "threat forgotten" - effectiveness did not last that long.
There are there are other states as well when the threat is "on file" in the immune memory. Another exposure months later influences this process. My guess is that the evolved heuristic is that "a threat that recurs more is worth remembering better".