His arms are a reminisce of Flex Wheeler's. Flex used synthol, and that's how it's broken down.
Flex used Synthol toward the end of his career, like 1999 and 2000, mostly in delts, but probably in his arms, which didn't have his trademarked striations in his bis. If you compare pics from 1993-1997 you see Flex was always "gunslinger" and had epic arms (even before he got his pro card).
Those who use synthol tend to not have crisp look to their arms. Which Phil has. You see striations and all, and the fact that one arm is bigger than the other (his left) shows that one arm is stronger, or gets more work out of them. If he was using synthol, why not try and correct the imbalance? Well, because he doesn't need to. Just like how Melvin Anthony has a double bi peak on one arm and the other doesn't.
Also Phil's arms have always have the same shape, even if you look at the his first couple of contests before he turned pro. Same shape, same detail, same striations in his bis and tris, they just grew, Thing is, he doesn't need synthol, those who tend to use it, know they have a weakness that they try to hide, and sometimes, like Ernie Taylor, become a "overwhelming" bodypart that they are known for.