The two variants of Kastoroff illustrate two ends of a continuum.
At one end, the party injured by the mistake (Kastoroff in the real case and the variants) will be significantly disadvantaged if the contract is enforced; while, if the contract is not enforced, the other party will be no worse off than if the mistake had not been made. Courts typically hold the contract is voidable in such cases (assuming the other conditions (1) - (4) are fulfilled).
At the other end of the continuum, the party injured by the mistake will be significantly disadvantaged if the contract is enforced, but the other party will be significantly harmed if it is not enforced. Courts typically hold the contract is not voidable in such cases (even assuming the other conditions (1) - 4) are fulfilled).
The difficult cases fall in between the two ends of this continuum.