Aside from what Sharon has pointed out, it really doesn't matter whether you get it or not and it certainly doesn't matter if it doesn't fit into your life of phonetics.
The Thigpen name has been researched by many all the way back to the homeland and it is what it is. Many, many names have changed over time whether on purpose to Americanize one's self or by virtue of illiterate people trying to put pen to paper. I have Thigpens on one side and Italian ancestry on the other. The transformation of Italian names is more difficult to "understand" than the Thigpen and is no doubt due to illiteracy and communication barriers. In some cases they just plain changed their names. So please, we don't need phonetics lessons. We know who are families are and whether you agree or not is irrelevant.