Home accents

I was watching a really interesting program called 'The Making of Me' on the BBC last night where the actor John Barrowman went out to determine what makes him gay. Something that I found very unusual was the fact that his normal accent is an indeterminate American one but that he has a Scottish accent when speaking to his parents. He was born in Scotland but they moved to the USA when he was very young so you would expect the Scottish accent to have disappeared.
I think that many people who move around have accent shifts. My accent when I am at home in Clare is more akin to the accents around me while my normal accent is very neutral to the point that many people have asked me if I am Canadian or something. However, I don't think my accent shifts so dramatically at home that anybody could mistake me for a different person but John Barrowman has two quite distinct accents.
Many Dutch people I know speak a dialect at home but they speak standard Dutch at work. You can hear the difference when they are speaking to family on the phone. Dialects are sticky though, words are pronounced differently and dialect-specific words are used. With different accents you are basically saying the same thing but with a slightly different pronunciation and intonation. That is why you would not expect the same person to have such a bipolar accent range.
One explanation might be that he was trying to 'keep the accent'. Some people are quite determined to retain their original accent when they go to another country. It could be that he kept his original accent around the family but adopted an American accent to fit in to his new environment. The surprise is not that somebody would do this but that there is so little mixing. He has no instantly recognizable Scottish accent in his American accent or vice versa. My own accent has traces of everywhere I have lived, I certainly don't have a set of accents to call on.