Why do I have a strange deep crease in the middle of my neck?

I've had this single crease going across between the middle of my neck ever since I can remember. I've been born with it probably... it's not the type of minor neck lines/creases that other people that I've seen that have do though is what I've noticed.

Here's what it looks like:
Why do I have a strange deep crease in the middle of my neck?
It's know that it's quite a deep crease and not the usual little creases and fine lines that that most necks have...
Why do I have a strange deep crease in the middle of my neck?
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