Macallan have in fact released an 18yo every year - there is no rule or practice of keeping it only to the odd-numbered years.
However, I understand that the 1985 heralded a major change. Previously, the 18yo releases stated on the label that the whisky was distilled in that year, i.e. the 1984 contained whisky from vattings that were all distilled in 1984.
However, the 1985 (apparently) states that the whisky was distilled in 1985 or earlier - i.e. there is now whisky OLDER than 18 years in the vatting.
I haven't seen this for myself yet, I only heard about it somewhere, so apologies if my assertions above are incorrect!