Usually BND performance would be very close to the TSP F Fund, but because of the Fed’s intervention in the bond market they are much further apart than usual. The reason for that is BND tracks the Barclays Capital US Aggregate Float Adjusted Bond Index which is identical to the Barclays Capital US Aggregate Bond Index (which is what the F Fund tracks) except that it does not include bonds held by the Federal Reserve. Because treasuries currently make up 1/3rd of the index, and the Fed currently owns about $2.5 Trillion in treasuries, that is a very significant change.
The Fed is likely to start winding down those huge positions towards the end of this year by not reinvesting funds received after treasuries mature, so the two indexes will start to come back into alignment. It will take awhile though – about 40% of the treasuries held by the Fed have maturities of greater than 5 years.