Keep thinking, you almost got it. Cruz needed evangelicals and the only place to get them was Carson. And only 4 votes per precinct. Easily done by his "Trump is unstoppable" threat.
Cruz didn't just win Evangelicals. That's an assumption by the pundits. Trump and Carson got their share in Western Iowa. Cruz won most of the counties in Eastern Iowa as well.
Also this "4 votes per precinct" spin from Rove is actually 9000 votes - more than what all except the top 4 vote getters actually got statewide.