The earlier responses that discussed inflation expectations have captured the difference between the price performance of traditional coupon bonds and infaltion adjusted bonds.
Here are the specifics. In the last year we've seen inflation expectaions rise. We now have a trailing inflation rate of about 6 percent--- that's why I reported in July that seniors were going to get the biggest COLA adjustment to their Social Security benefits in 25 years come January.
Because of that increase, TIPS were priced up so that their non-inflation yield approached ZERO on shorter term instruments. Why? Because a 6 percent adjustment to principal was still way better than a 2 percent coupon on a short term Treasury.
In July the CPI increased a sturdy 0.5 percent. But in August it dropped by the same amount. And I don't think anyone will bet against me when I say September is likely to be another negative to neutral month for the CPI. That means the CPI component of TIPS return is declining so the future "return" is likely to be smaller. Hence, the price will fall.
Global inflation protected securities, like the WIP ETF, work on the same principles as TIPS but they are priced in other currencies. So you have a lot more going on--- currency differences, inflation differences, and basic coupon differences.
We've now had quite a few years of fundamental decline for the dollar. But the dollar is still the global settlements currency and the world is short of cash. As a consequence, the dollar has been very strong. Whether it will continue to be strong will depend on which governments resort to printing money.
Scott