There are two sides to this matter.

On one side they want to check whether you have actually draw such salary or somewhere near that range. If previous job someone if drawing 2.5k then the new job the person is asking for 6k to 7k, now something is wrong here. On the other hand you will be on the losing end, if they try to go lower than your offer. It is up to your negotiating skills. To me i will not even give them.