I thought refer a friend had ended?

I referred some family members to flying club last summer as were flying to Cancun together and thought I may as well cash in on their miles. All well and good and lots of miles banked. However my parents flew to Montego Bay last Friday and I seem to have had 1000 miles added to my account for each of their flights! I'm not complaining about the 2000 free miles but I can't see why this has happened. They have yet to return so I'm not sure if the same will happen then but I will wait and see I guess.