As long as all of your estimates come from reputable, licensed, and insured contractors there are only a couple factors that could contribute to the difference in pricing.
Attention to detail during preparation is one of them. All of the steps from the prep process are covered up once your home is painted, and this is where many people take shortcuts in order to complete the work in less time, charging a lower price.
The preparation makes or breaks the whole job. This is why it's critical that your receive a well written, detailed explanation of what each paint contractor proposes to do. You need to be sure the proposed scope of work is the same from each contractor in order to compare apples to apples.
Some may also use less expensive, often inferior products to keep their prices below the competition. Make sure each contractor outlines what products they are specifying to use on your property.