Because of the wide range of types and maturities of corporate debt securities, as well as the range of creditworthiness of its issuers, corporate debt securities have widely varying potentials for return and risk profiles. For example, commercial paper issued by a large established domestic corporation that is rated investment-grade may have a modest return on principal, but carries relatively limited risk. On the other hand, a long-term corporate note issued by a small foreign corporation from an emerging market country that has not been rated may have the potential for relatively large returns on principal, but carries a relatively high degree of risk.

Individual shares of the Fund will be listed for trading on [the Exchange] and can be bought and sold in the secondary market at market prices. The market price of shares will fluctuate in response to changes in the value of the Fund’s holdings, supply and demand for shares, and other market factors. ProShare Advisors cannot predict whether the Fund’s shares will trade above, below or at a price equal to the value of the Fund’s holdings. Differences between secondary market prices and NAV, the value of the Fund’s holdings, may be due largely to supply and demand forces in the secondary market, which may not be the same forces as those influencing prices for securities or financial instruments held by the Fund at a particular time. Given the fact that the Fund’s shares can be created and redeemed in Creation Units, ProShare Advisors believes that large discounts or premiums to the value of the Fund’s holdings should not be sustained.
There's a lot of ins and outs which can get confusing in arbitrage trading. We won't go into the technicals of why futures contracts trade at a premium to spot price. You can read a full explanation here. If it's not of interest to you, all you need to know is that there's a tendency, the further out in time the futures contract expires, for the premium to spot to be higher and higher in nominal percentage terms.