This is true. But it's also true that if you push, you will hit a point at which the editor will say, ''I can't do that,'' and will mean it, and if you don't give in you'll lose the sale. Where the point is varies wildly with the exact circumstances. Now, if you're a first novelist, the editor has probably made a conservative offer. Most first novels lose money.
Most first novelists work cheap, so as to break in, and they still mostly lose money.
In between, you can try to negotiate for more. You may not get it, but it shouldn't hurt to try. That's the advance. The advance is important. It's what you get up front, the only thing you're guaranteed.