  :: dear Kiumars.. you did not read that statistic carefully.  It is from 1991 , Mordad.  In it the mother language of all mothers who gave birth and received birth certificates was asked.  About 20% of mothers spoke Azeri, 10% Kurdish, 46.2% Persian, 9% Northern dialects (close to Persian), 9% Luri (which is SW like Persian and mutually intelligble).  So the statistic was about the mother tongue of all babies that were born during that month.   It is the most accurate  one I know about and the results match provincial statistics.  About Kurds, I know the average Kurdish family in West Azerbaijan has 5 kids where as the national average is little less than 2.   Also many Iraqi Shi'ite refugees came to Iran during 1990 and 1991 and I am not sure how many of them were given citizenship and this could explain the 5% Arab (as well as the higher birthrate).  