When I was transliterating my family members' names into Katakana, I asked my teacher how she would write my brother, Andrew's name. She told me to write it as アンドリュ Andoryu, but I don't understand why it wouldn't be Andoru? Can anyone explain this to me?
I'm fairly new at japanese so I'll try to do my best.
You write the words the same way you pronounce them. If you write you brother's name Andoru, it means you pronounce it /Andr/ (kind of).
I hope my explanation was somewhat clear .
In part it depends on which you choose to write, in part it is determined by convention. It would be against the convention to transcribe David Beckham as デイヴィッド・ベックハム as there is already a fixed name デビッド・ベッカム.
As for Andrew, there's already a fixed アンドリュー, so you can transcribe it as アンドルー but people aren't used to seeing it.