Religion and the number of adherents.

Today in class we had gotten our exam grades and the essay back. Our question for todays blog is are all of the adherents numbers the same since a year ago. Islam had 1.5 billion, Hindu had 1.1 billion, Jewish had 14-18 million, Buddhism had 500 million- 1.5 billion. These numbers had been estimated from about a year ago. Todays answer says Islam had 1.7864 billion, Hindu had 1.155 billion, Jewish had 1.54 million,  Buddhism had 5.467 million. These numbers have changed by both increasing and decreasing. For example Islam has has increased, Hindu has stayed about the same, jewish had increased, Buddhism has increased. Some of them differ in increasing or decreasing. My question would be why does some increase and some decrease. On the last site we had to look on is about roughly the same facts. Christianity is the biggest followed religion. It is said that christianity will be the most popular religion. I am having a little bit of trouble understanding why they would decrease because if you believe so strongly on one god or a certain culture how can you change. Another thing is how can one increase because aren't you supposed to stay with that religion because you believe so strongly in it. I would like to know more about how you are able to leave and go to another religion.