What should be the difference between low end and high end when getting estimate?

We recently got quote from several companies for a software development. And most of them have about 10% difference between low end estimate and the high end. But some companies are going as high as 45% possible increase. And the conditions and what needs to be developed are already discussed and agreed upon.

So if everything is already discussed and agreed, why going as high as 45% potential increase? And what is the normal margin/percentage if everything is agreed by both sides?