This is just speculation on my part. High volume NAS brands dump lots of barrels together. The barrels are not exactly the same. They are all at least 4 years older. Some are probably older or "honey barrels" added to try and keep the overall brand profile consistent. My theory is somehow it all didn't get completely mixed together, and your bottle got an extra large dose of one of the better barrels. Another idea, maybe your bottle was one of the first ones bottled directly after they finished bottling a run of one of their high end brands.
My experience with variation was with Old Fitz BIB. I got a 750ml that was extra good. It tasted like a 100 proof version of Larceny. I went back to the same place and got a handle. When I opened that, it was just the same old run of the mill.