Presumably the additional givers are not donating for selfless reasons, but for the rebate. I hear you say that there is something odd here. Why would people give money away to get less back in the form of tax deductions. Well of course they generally will not. But the deduction reduces the comparative cost of giving to charities and thereby increases the amount they get.
Economic rationalism strikes again.
It's odd that those who rail against our consumerist society and the influence of economic rationalism do not call for the scrapping of charitable rebates. Presumably it has nothing to do with the preponderance of the great and the good on charity boards. There is nothing like a spot of fund raising to bring out the best in a CEO's spouse. For some reason working for a living does not have the same kudos as working for a charity. At any rate, people are now giving for the pleasure gained from giving to a worthy cause and because part of that pleasure is almost free.