many brewers tweak recipes all the time, for a variety of reasons. Most frequent is to iron out season-to-season changes in hops, malt, etc. Different growing conditions mean that year to year characteristics can change (just look at alpha acid content fluctuations, for example - those fluctuations also apply to other characteristics as well). Or maybe one of the specialty malts used has been discontinued, or produced using a different process. Heck, my local water changes in character and mineral composition through the year
Really, the only beers that are 100% consistent batch to batch, year to year, and decade to decade are the light american-type lagers.