When I was in elementary school, there was one teacher who would always write the number 7 with a line through the middle, and I never knew why. I just assumed it was a personal preference and I started doing it for awhile myself.
Well, I stumbled upon this slide show on the Arabic numbers that gives an explanation on why all of our numbers (1 - 9, and 0) are shaped the way they are and that also answers my question on the number 7.
http://www.slideshare.net/nzar/numbers-1218076