Just some other bits on this;
Cask strength has to be th le strength of the spirit once it leaves the cask, wih no furher alteration. Something like a talisker 57 north is not cask strength despite being 57% - its he strength its bottled at to give range consistency.
Im not aware of different casks giving different strengths (1st vs refill) but the biggest contributer to strength is relative humidity of the maturation location: over years alcohol will evaporate from the casks, this is because oak is porous (angles share). Places with high humidity loose less alcohol than those wih lower humidity - he wood expands in hiher humidity areas so less evaporates throug it, there is also more water in the air so less alcohol moves. This is why karuizawa will be 62% after 40 years and the 37 yr glen grant i have open at the moment is only 47.8% - lower humidity.
Some american whiskies actually increase strength as they age!
Other factors also include the location in the warehouse that the cask was in and the air preasure and temp too.
Some old scotch will drop below 40%, its still whisky but cant be bottled as such as it has to be over 40%. Vatting wih other whiskies can be used to raise the strength.
This whole thing is why adding water to whisky in your glass is bad - most have been diluted with water from the distillery, sticking tap water in it from the other side of the world will ruin it