Diplomacy is not an exact science and posturing always takes place and sometimes positions change. It's just that this particular negotiation is under the microscope so we see the process step-by-step. But this sort of thing goes on in every negotiation.
I read that they were looking to purchase 20% enriched Uranium from abroad. This is good for nuclear power plants, but not capable of being used for a weapon. It's still worth pursuing as Iran still has a long way to go before being able to make a weapon. It costs little to talk. If it yields nothing, then military action is always an option and it make sense to wait till they're closer as striking then really sets them back. It also gives the appearance that all was done to prevent it. It may be the case that Iran is stringing the West along, but don't realize we're doing the same in a way.