In the case of Glenfarclas, it's probably easier for them to do the minor amount of tweaking to get to 60% than it would be to deal with variable strengths on the label and legally (tax). It's just easier if a continually-available expression doesn't jump all over the place. By contrast, each batch of a'bunadh is a new release. Balvenie 15's label always changes, with cask #, etc, so that's not an issue, but tax still is, and as far as every wholesaler in the world is concerned (LCBO, etc), it's a single product with a single catalog number. I think probably it was never intended to be cask strength, just have a little more body and kick, and maybe be above the 46% threshold. Why 47.8%, precisely, who knows?