Why is "4a IRA distributions" in 1040-SR different from "4b Taxable amount"?

For years, the amount in "4b Taxable amount" is always smaller than the amount in "4a IRA distributions" on my 1040-SR.  But 2023 "IRS 1040 (and 1040-SR) Instrubtions" does not seem to tell me the reason why.  I always do Married Filing Jointly and enter 1099-Rs in Turbotax Premier.