I agree with Ishorton99.
Perhaps it �s easy to understand if you consider what the situation is in each case :
You are not spending your vacation in Seattle, but ...
If I were you, I would be spending my vacation in Seattle.
I am flying to Detroit, but ...
If I weren �t flying to Detroit, I would have planned a trip to Vancouver.
I am not taking this exam next week, but ...
If I were taking this exam next week, I would be high-strung.
He had been driving fast, but ...
If he hadn �t been driving fast, he wouldn �t have crashed.
I hope this explains the use of the continuous in these sentenses.