Counterfactuality is just the extreme of unlikeliness - zero probability.
If one of your colleagues turned up to a serious business meeting dressed as Mickey Mouse, it’s likely you’d remember the funny incident a month later...
A hypothetical situation which could (however unlikely that might be) occur in the future.
If one of your colleagues were here now, dressed as Mickey Mouse we'd both wonder what was going on.
A hypothetical situation which does not exist. The situation of your colleague being here now, dressed as Mickey Mouse, is counterfactual.