We can believe them on this. Really. Obama is the president so we know the Iranians won’t lie.
It seems that the Iranians do not think that they will get the Uranium back in a processed form, and we can not trust them to ship the LEU. Why not exchange the Iranian LEU with the equivalent amount of fuel in several shipments?
The advantage for the Iranians would be that they will get it faster and for the rest of the world that it would be removed from Iran. It could even be OK to trade them more than the equivalent amount of the Uranium that they are shipping.
For anyone who doesn’t know, this is part of a plan that was to have Iran ship it to Russia, where it would be made into fuel rods, then shipped back to Iran. They have repeatedly said they dont want to do this... It apparently was an attempt to let them have it for civilian use, as some people continue to pretend that is what Iran wants.