Partner awards are almost always capacity controlled and operate as saver awards for redemption.

An interesting situation occurs when the airline allocates inventory to a partner airline, say United Airlines allocates 2 seats for a particular Chicago-London route to US Airways. United may have given out all its award seats for the flight and then a passenger redeeming UA miles may only be able to get a higher mileage unrestricted award.

US Airways may still have its allocation of 80,000 mile awards while a United member is being asked to pay 180,000 miles for a standard, unrestricted award.

This has happened to me when I wanted a Star Alliance award using UA miles and I could get the identical award for 80,000 miles through ANA, but it was unavailable using UA miles because ANA still had its allocated inventory whereas the saver inventory was gone with UA.

Similarly, I have found UA flights on ANA available for lower miles when the UA saver inventory was gone, but the ANA saver inventory was available.