Short Answer: Not all Bosniaks are Muslims.
Long Answer: Let's start at the beginning, just over a thousand years ago. Originally there was a single south Slavic ethnic group living in Bosnia and the surrounding territories of what used to be Yugoslavia. When the Ottomans conquered Bosnia, they introduced the "millet system" which divided everybody on the basis of religion. Due to this imposed division, members of different religions began to see themselves as distinct ethnic groups: Catholic Croat, Muslim Bosniak and Orthodox Serb. At that time "Bosniak" was synonymous with the Muslim millet in Ottoman-controlled Bosnia. All Bosniaks were at least nominal Muslims.
Then the Ottomans lost control of Bosnia to the Austro-Hungarian Empire and the millet system was formally abolished. Bosniaks were now free to convert to other religions without fear of punishment from the Ottoman state. Some reverted to the Christianity of their forefathers; these tended to be "crypto-Christians" whose ancestors had formally converted to Islam to gain the taxation and social benefits of being a Muslim in the Ottoman Empire, yet secretly practiced Christianity. This process of Christian reversion is still going on today in Bosnia, Kosovo and surrounding regions:
http://www.economist.com/node/12868180
During the Communist era, many Bosniaks became atheist, particularly those educated in Communist-run schools.
Anyways, due to family, social and cultural associations, these non-Muslims feel a closer kinship to Bosniaks than they do to Croats or Serbs (even if they are Christian), so they identify with that group even though it is historically and majority Muslim.
There must be, even Arabs have christians who never have converted since Islam's advent
http://en.wikipedia.org/wiki/Bosniaks
who cares.....?
