It's all very simple:

Since 1977 membership in the G7 has been fixed, although 15 developing countries' leaders met with the G7 leaders on the eve of the 1989 Paris Summit, and the USSR and then Russia have had a post-Summit dialogue with the G7 since 1991. Starting with the 1994 Naples Summit, the G7 and Russia have met as the P8 ("Political 8"), following each G7 Summit.
http://www.g7.utoronto.ca/g7/what_is_g7.html

I think 'political' in this case refers to countries with loads of dangerous weapons.