The reason why Muslims prefer to call some one revert than a convert is because of a Hadith (Saying) of the Prophet.

The Prophet (peace and blessings of Allaah be upon him) said: “Every child is born in state of fitrah (the natural disposition of man), then his parents make him a Jew or a Christian or a Magian.” (Narrated by al-Bukhaari, 1296; Muslim, 4803).

In other words, we are all born as Muslims then our parents convert us to Christianity, Judaism etc.
That's the reason why they call some one who accepts islam as a revert because he/she used to be a Muslim before your parents convert him/her into Christianity or whatever religion it be.