Preacher’s kid here. Many people tithe more than 10% of their income. I do myself. The odd thing about tithing is I never seem to miss the money. Tithing in excess of 10% is not necessarily an indicator that the person is under reporting income to the IRS - although I’m sure the IRS will automatically assume so.
You are correct that it doesn't necessarily imply under-reporting but it can which is why questions like this are asked.