I agree that there is a difference. But it's possible to have two residences (homes). And if you do have two residences and sell one then you now are left with an 'only or main residence'That is not correct. There is a difference between occupation and residence.
And the current residence automatically became 'the only or main residence' at the moment the other was sold. The fact that the op intends to sell it doesn't affect that determination.
Before a property can be an only or a main residence it first has to be a residence. Just because a person owns a house does not mean that he (or she) owns a residence.
The OP doesn't make it clear whether they had two homes or not.