The simple answer is that when an ‘e’ is included in whisky it is due to where the liquid originates from. Traditionally ‘whisky’ comes from Scotland and ‘whiskey’ comes from Ireland, however as the category has spread across the world this has become more nuanced; in general whisky is spelt with an ‘e’ in the USA but without an ‘e’ in Canada.
The Irish started to spell whisky with an ‘e’ in the late 19th century, to ensure they were seen as different to their main Scottish rivals. Brands that spell their spirit that way can usually trace their influences back to the Emerald Isle.