So, as a new employee out of college, if someone offers 100K, and you reply with "130K and we have a deal" are you going to be 1) laughed at and rejected 2) rejected 3) considered? I've always been one to just accept whatever -- "Yeah, that sounds good!" -- but I'm consciously preparing myself to avoid this tendency after I finish grad school.
The decision to hire is expensive, if they've already made an offer there is usually some room for negotiation either through salary or something else depending on the company (equity, time, travel).
If you were offered 100k and said that you'd love to work at the company, but would be much more comfortable at 120k they'll probably move a little bit just to close you.
Worst case is they'll get back to you and say that they're not able to move from their original offer in which case you can accept without worrying about it.