The effective tax rate is tax expense (income statement) divided by pre-tax income (EBT, income statement).
The cash tax rate is cash taxes paid (cash flow statement) divided by pre-tax income.
They’re different because of all of that wonderful junk you learned at Level I about deferred tax assets (DTAs) and deferred tax liabilities (DTLs).
Because earnings – net income – comes from the income statement, you use income statement accounts to project it.
Because cash flow comes from the cash flow statement, you use cash flow amounts to project it.