Many thanks ! I now figured it out. It is a very strange system here in spain (not surprisingly !!!).
It looks at the variation in IPC from the month you started your contract until the next 12 months. If I get it correctly, If you sign in a month with a low IPC, you have bad luck, if you start in a month with a high IPC you are lucky.
I would have expected the average increase in IPC during 12 months ! Which is the case in most coutries I know of. But hey. Mine is 3, the average 1.8 - bad luck. I will still try to negotiate now though. In this economical climate it is not the time to increase the rent, and mine is already very high.
Thanks for your help !