Comparing two numbers proves nothing, if you don't look at the source of the funds. The funds backing USDT could all be borrowed from a bank on the condition that they are never withdrawn, and then in turn used to lend USDT on Bitfinex at a much higher interest rate. Or worse, the source of the funds backing USDT could be proceeds of selling Bitcoin that had been purchased with fraudulently printed USDT. Just comparing the numbers would do nothing to eliminate these possibilities.