Hello Manu,
I just finished year 2006 in my career and when I started year 2007 I lost 4 positions in rankings (dropped from 11 to 15). Why did this happen? Last tournament I played was Paris Masters. After that, I didn't play ATP Finals nor Davis Cup either. These were the last tournaments in the year. Davis Cup gives no points and the guys who passed me in the rankings were not on Finals. Anyway, even if they were in Finals, my ranking dropped only in week 52. That doesn't make any sense, since there are no tournaments scheduled from weeks 47 to 52. Is it a bug or is there another explanation?