Global central bank gluttony?

Is it behind the treasury rally?   RBS estimates that global demand for high quality bonds is $1.2 tn while "net" supply is only $600 bn.  Meanwhile, U.S. banks increased treasury holdings by 23% in the first quarter. So there’s lots of cash and a “net” shortage of bonds.  The word “net” is key, since the too meager supply is net of the 60%+ of all securities being issued in the world that are being purchased by central banks.  (FT Weekend, p 12)