Batch Differences - One Good, Others Not So Good

[Islay Peat]

I've had Glenfiddich 12 on a handful of occasions so far and have only in one instance thought of it as palatable for my tastes. That one particular bottle has tasted good every time, but others have only been drinkable, but not enjoyable for me.

The particular bottle of Glenfiddich 12 that I liked is older, possibly 15 years or more, and if I remember rightly, the label and packaging are different from they are now.

So the differences I'm noticing, are they more likely to be due to batch variations, or has Glenfiddich had a general change in the distillation process that accounts for my liking one bottle, but not another?