It seems I was depending on the order of GrantResults returned by a GrantItemsToUser being the same as the ItemIds passed into GrantItemsToUser. But either I was crazy, or that stopped working recently (and I'm still crazy for expecting the order to match). (The order may have changed when I recently renamed a bunch of my items)
It would be nice if the order matched.
It would be nicer if the docs specified one way or the other. I know, I'm crazy for expecting this. I thought it was too good to be true when it seemed to work before. I know, you'll go crazy documenting every crazy thing I might have counted on.
You could just show the sample response in a jumbled order.