Apples to Apples
You need to be sure that you are describing the same thing to each company you are speaking with. Often your ideas about an application will change during your conversations. If your requirements are not one hundred percent nailed down prior to asking for quotes, each company may have their own understanding of what it is you would like to build. This can be an issue because their concepts of the application may not be the same.
Companies should ask a lot of questions about your project to get a complete understanding of what you would like to build. Sometimes these questions will trigger thoughts about the product that you may not have considered before. This is great for your understanding of the project, but other companies you interact with may not receive the new information. The questions asked and your answers can have an enormous influence on the quote you will receive.
For example, a company may inquire about your launch plan for a mobile application, whether you plan to launch regionally or nationwide. If you plan to launch regionally at first, this could decrease the effort to build your first version of the application. If the other companies you are speaking with assume you are launching nationwide, their quotes would reflect that.
During discussions about your product, companies may have suggestions for features or recommendations for development methods that could affect their quote. For example, a company may suggest either a Native or Hybrid app model. A Native application for iOS and Android can provide a better user experience but could also be a larger effort. Whereas a Hybrid app enables you to build applications for iOS and Android from one codebase. If a company suggests a feature you like and you agree that it should be in the quote for the application, other companies may not have considered this feature and their quote would not reflect the associated cost.
One way to ensure that companies are quoting the same thing to provide a set of requirements or a specification to each company. If you can provide a set of requirements and a complete description of the application in writing it avoids scope creep and some of the variances you will get when companies quote a project based on a verbal description.