I'd wager we're dealing with lesser-quality casks here, finished in first-fill sherry to put lipstick on a pig, as it were. Note that the bottle carries no information on the precise mode of maturation, only that this has "a big sherry influence" (no argument from me here). Also somewhat hot, which, again, is a bit weird for the stated age.
I've grown very disenchanted with the whole "regional malts" series (some of the NAS versions were so immature as to border on the frivolous). This isn't bad per se, it just does not deliver what I'd expect from a sherry-matured 21-year-old. Not worth the asking price.