If it was about a price, there could have been one Q/T (etc) seat left and that is all was all.

The best thing to do in your case, and there is no downside at all, is to place the reservation "on hold until midnight tomorrow.". This will guarantee your flight and fare, even if it crosses over advance purchase requirements.

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NW is a PITA w/ this procedure... you couldn't do it on their old site, maybe you can on the new one. You used to have to call them and hold it on your Credit Card, and if you didn't call to cancel, then it would book... w/ CO if you don't call (or cancel online) it just disappears and there are no problems.