A question about Tether

[Beerwizzard]

Hey guys!
Right now I'm looking on USDT (Tether) chart. On their website devs say that it is always 1-to-1 backed to traditional currency (in this case USD) so I should see a straight line on their chart but if you take a look on the graphic you will see that in may it was about 92 cents for 1 USDT. Can anyone tell me what happened on that time? I did not followed it in may.  The changes are not serious but I just want to be sure that there is nothing to warry about.
https://coinmarketcap.com/currencies/tether/#charts