From the F Fund Fact Sheet available on tsp.gov:
BlackRock’s U.S. Debt IndexFund —
The F Fund is invested in the U.S. Debt Index Fund. Because the U.S. Aggregate Index contains such a largenumber of securities, it is not feasible for the U.S. Debt Index Fund to invest in each security in the index. Instead, BlackRock selects a large representative sample of the various types of mortgage-backed, U.S. Government, corporate, and foreign government securities included in the overall index. Within each sector, BlackRock selects securities that, as a whole, are designed to match important index characteristics such as duration, yield, and credit rating. The performance of the U.S. Debt Index Fund is evaluated on the basis of how closely its returns match those of the U.S. Aggregate Index.
They don't match for that reason. Hope this helps!
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