the answer may surprise you.
many etf's are allowed to put up their treasury bond holdings as collateral for money to invest else where like money market instruments from banks.
TLT has it in the prospectus that they can do this with up to 1/3 of the funds assets .
they can also buy up to 5% in commercial paper bringing the total to 39% of funds assets in commercial paper.
so how much do they actually loan out ?
according to this 2012 video 36% of assets were not in treasury bonds but in commercial paper .