Win7 64bit has a file titled "Program Files (x86) " this is for when you install 32bit program and where it is installed into. So you can use most 32bit programs in a 64bit sys...so all x86 refers to is 32bit programs..I can't say as I have noticed Win7 x86 titled as such but all it would mean is a 32bit sys...Hope this helps..
My guess would be from the old computers number system (286, 386, 486). The first Pentium was in the 586 series if I remember correctly. If you notice the first number is the only difference therefore making them all in the x86 class.