Entering $20K in Box 12, code BB from W-2 form has no effect on the federal income tax. Why? Isn't a contribution to a 403B tax deductible?

Should the $20K entered in Box 12 BB reduce the taxable income on the W2 form? If I replace $20,000 with $0 (as a test), there is no effect on the Federal Tax Due. I thought a 403B contribution would reduce the taxable income.