In the periodic table, why are groups 13-18 grouped differently

Why is halogens group 17, when sometimes people call it group 7? I get confused with this please explain why.

Group 13-18 are all non-metals. There are 8 families in the periodic table.
column 1 = family 1 (alkali metals)
column 2 = family 2 (alkaline earth metals)
column 13 = family 3 (boron family)
column 14 = family 4 (carbon family)
column 15 = family 5 (nitrogen family)
column 16 = family 6 (oxygen family)
column 17 = family 7 (halogen family)
column 18 = family 8 (noble gases)

if its hard for you to remember. try this short form "aabcnohn" :)
if you are still confused, visit this link : http://library.thinkquest.org/C0110203/char.htm

group 17 came from the numbering of the groups from left to right and you can only use arabic numerals while group vii is from their properties as halogens (7 electron in the outtemost shell) and can only be writtrn rman numerals