They used the significantly lower rates of the debt tokens to buy them back on the market, and thus lower their active debt without paying the full 1:1 price. In other words, people who put their coins in the market, will never get 1:1 as WEX will buy these debt tokens at current rates. You will only get paid the full 1:1 ratio (if ever) if you patiently keep them in your account. It was to be expected, and honestly, you can't blame them for buying back their debt at far lower rates. All tokens move below the 0.6 level, and some even below 0.5 currently.