The easiest approach is to speak about how you thought other aspects of the offer might have been better which is why you used the salary number you did. Those other aspects might be (benefits, equity, paid time off, flex hours or location of work, etc). You can see a full list of these things here: Lifestyle Calculator

In the future, always use a range when being asked about salary saying something like: I am looking at offers between x and y (where x is the number that would be the lowest acceptable if everything about the job other than salary was perfect and y is where you dont like almost anything else about the offer but if the salary were this high you would take it).