Theory #7: Cobb entered a deeper level of limbo.
This theory opens up the possibility that Cobb entered a deeper level of limbo. Why so? Let’s return back to the sequence at the end of the film that showed Cobb trying to save Saito from limbo. Remember that Saito picked up the gun on the table, and then the scene was immediately cut to Cobb waking up at the “airplane level”? What exactly happened in limbo? To explain this theory, the inference to take is that Saito shot Cobb, (incorrectly thinking that by doing so, Cobb would get out of limbo), causing the latter to enter into a deeper level of limbo.
This deeper level of limbo manifests itself as the “end sequence” showing Cobb clearing the US customs and reuniting with his children. The outcome of the spinning top could be interpreted as such: Before the top could stop spinning, the screen fades to black. A possible explanation could be that the sedative wore off at that time, and Cobb woke up at the “airplane level” from the deeper level of limbo.