Like most things in 40k, that's up to you. Maybe the Genetor gave him a number because he's a cold and unfeeling jerk. Maybe what the Genetor *really* wanted was a son, so he gave him a name and raised him like a person. Maybe the Genetor only ever gave the poor sod a number, but he chose a name for himself, because he's determined to rise above the circumstances of his birth. Maybe he was mass produced as part of a huge batch of tithed elite soldiers, but has a name anyway because research shows that human names help to increase unit cohesion.