Ialmisry's right. Except that the Israeli government claims that the number of conversions from and to Christianity and Judaism is relatively similar. So what seems to be happening with the phenomenon you mention is that a big number of those Israeli Christians are actually already Christian before they immigrated. They immigrated based on the fact that they have partial Jewish ancestry, Israeli law gives an easy opportunity for people with partial ancestry to immigrate, and there are a significant number of Eastern Europeans with partial Jewish ancestry due to the fact that Jews are one of the significant ethnic minorities in Eastern Europe, next to eg. Rusyns, Tatars, Gypsies, Russians outside Russia, and Germany's Slavs.