The point of serial number control is obviously to track the serial numbers of your items.
What you have done is set the system up to allow you to do this. I will presume you have registration turned off. You then entered 10 and entered the serial number on the line. Because you are manually controlling the serial entry YOU are telling the system what is happening, YOU have told it that they ALL have the same serial number, which is possible as mad as it sounds.
Therefore you need to control this, either insisting on registration to perform the serial number entry across the 10 (but you can still enter one number) or so use the serial number batch groups with a quantity activation of 1 to generate the number per 1. When the system generates it it does so to its own rules, When you allow manual entry it does it to your rules - there is no restriction on a 1 for 1 serial number creation when you do it manually.