The document request and the time frame given depends on the officer reviewing your application. If you send it in quickly, your application will progress. Could be that the officer is waiting for this last document before he/she makes a decision. Some officer want things to move quickly and give less time because when you submit a document after a long time, they will have to review why your file again as to why was the document requested. In 7 days the file is still in the officer's memory and he can quickly process it to the next stage.
