On June 13th I asked Tether out of curiosity why they were under collateralized:
https://twitter.com/FollowTheChain/status/742373658019012608

To my surprise a few moments later they simply changed some names in the Transparency page.
"Bank Reserves" became "Reserve Balance".
"Total USDT in Circulation" became "Total USDT Authorized".
The middle column (what is now "Issuance Pool") had a negative value and I think it was named something like "Excess Reserves".
Compare the USDT and EURT lines and you'll see the math doesn't add up... They blatantly removed the minus sign!

The whitepaper doesn't mention an "issuance pool" or "authorized" USDT. The document clearly states:

At any given time the balance of fiat currency held in our reserves will be equal to (or greater than) the
number of tethers in circulation.

I couldn't find a recent capture from that page. Here's an old one:
http://web.archive.org/web/20150921165318/https://wallet.tether.to/transparency
The page then got removed from main search engines, and a google cache search redirects you to the Press page:
http://webcache.googleusercontent.com/search?q=cache%3Ahttps%3A%2F%2Fwallet.tether.to%2Ftransparency

How weird is that?? Why all this trickery?

A wild guess: maybe they really are under collateralized. In this case, issuing a lot of USDT would be a way to:

1. decrease the chances of a fractional reserve being exposed (the deficit remains the same, but becomes a smaller percentage of the USDT in circulation);
2. try to profit from the spread by passively trading with large walls at a premium (providing liquidity, stimulating higher volume), and hopefully making enough money to recover to full reserve.