Why do some people write number 7 with a line through the middle?

If you look at your keyboard right now you will see number seven looks like this ( 7 ) . Even look at your mobile , home phone or calculator  they're all the same . However many people ,including my self write it with a line through the middle. Have you woundered why ?

I have been searching in the internet for the answer and my search lead to two reasonable reasons .

1- Some people say that its written with a line in the middle to distinguish it from number one (1) .

2- Other resources said that if you go back to the reason why number one is (1) and why two is (2) , etc is about the angels that could be perfomed with the shape it self . Therefore 7  had to had a line to form the 7 angels.  

So was the seven with line through older than this seven (7) ? Dose different parts of the world use different 7's ?

Still i can't figure it out..i will update the article if i have any new additions to this subject .  

Remark: The numbers we all use (1, 2, 3, etc.) are known as "Arabic" numbers .