Apple has stopped signing iOS 10.3 firmware version and iOS 10.2.1 for all the iOS devices like iPhone, iPad and iPod Touch which means that users can no longer downgrade or restore their iOS to that version using iTunes officially.
Now you can’t downgrade or restore to iOS 10.3 or 10.2.1 on iPhone, iPad and iPod touch, as Apple stops signing both iOS 10.3 and iOS 10.2.1 firmware version. So you are basically forced to upgrade to iOS 10.3.1 directly. If you have accidentally update your device via OTA, then there is no way to downgrade back nor restore to iOS 10.3/10.2.1 now as Apple stops signing them.
Apple always gives two versions of iOS to restore whenever it releases a new version. So that people have ability to downgrade their devices to the previous version if they are not satisfied with the new one. But after some time, it stops signing the previous version.
You can also visit this link https://ipsw.me to check the signing status of Apple firmware.
Note: If you are on lower iOS versions like 10.1, 10.1.1, 10.2, 10.2.1 stay away from update because jailbreak is only available on lower version of iOS. If you are a jailbreakers, do not upgrade to iOS 10.3.1 as there is no jailbreak for iOS 10.3.1. So it’s better to stay on older version of iOS for now.