And to go along with SM, I think some people just prefer the full version of their name as adults, even if they weren't called that as a child. Everyone called my parents by nicknames their entire childhood. In fact, they still call them those nicknames. My parents hate those nicknames and never use them themselves. They just prefer the more "grown up" sound of the full names rather than the kiddy versions (and they do have names where the nicknames sound like children's names.)

I could also imagine that Gregory might have some want/need to be taken seriously and heard because of his background, and might like the stateliness of the full name. Philip, by contrast, loves to fit in and does it like a fish swims. So in my head, when he got to the US and started hearing other regular guys calling him Phil, or calling each other Joe and Steve, he would've liked the idea of Phil.