What is, and is not, a variety is something of a 'grey area'. Decades ago, 1961 halfcrowns without the engraver initials EF on the reverse were considered to be a variety. However as soon as it became clear that it was the result of a filled die, it disappeared from the list of varieties overnight. Only very rarely are such things classed as varieties, the exceptions being things like the 1946 ONE' penny. They are accidents of the minting process rather than design changes and therefore only collected by a small group of people.