Downloading

Hi ive noticed that when i download lets just say windows 8.1 64 bit and then the next day download it again it re downloads every file saying the remote file is newer than the one stored on my usb stick. Surely it should just check to see if its there and if not download it and skip any other file. It makes downloading updates very long winded. It does this for all versions of windows and office, and as office is a 4 gig download this is very annoying. I some times run the update on a different computer but the result is the same no matter what files are there it re downloads them all ......