Conservatives are more religious, and mostly tithe. All these surveys do is take IRS data "showing the value of charitable deductions claimed by Americans taxpayers." What the IRS may mean by charitable, and what most people think of as charitable, may not be the same thing.
For instance, a local fundamentalist church may spend the bulk of its resources degrading and attacking other faiths, insulting gay people and leading crusades to strip people of their civil liberties. They may never feed the hungry, clothe the naked, or comfort the afflicted. Yet in IRS terms they are a charity no matter how uncharitable they may be.