USD OIS curve. Why is the the bid bigger than the ask for maturities > 7 years?

I was wondering why the bid is larger than the ask for maturities bigger than or equal to 7 years?

If i export the screen to Excel i can see how the bid and ask swap rates are calculated. For the bid it is essentially just taking the bid Par swap rate for USD 3m LIBOR (=:K) and subtracting the bid par swap rate for a LIBOR-fed funds basis swap (=:s), with some adjustment for different payment frequencies and daycount conventions it creates a synthetic Federel funds fixed-for-floating swap i.e

(LIBOR- K) - (LIBOR-(FF+s)) = FF-(K-s)

 (bid)    -      (bid)     =  (bid)?


Bloomberg quotes (with some modifications) K-s on the screen in the 'bid' column for maturities bigger than 7. But why would you subtract bid for bid since you are 'selling' the LIBOR-ff basis swap. Shouldnt you take the bid from the Par swap and the ask from the LIBOR-ff basis swap to get:

(LIBOR- K) - (LIBOR-(FF+s)) = FF-(K-s)

 (bid)    -      (ask)    =  (bid)