I regret that I can't confirm your problem. I reluctantly retrieved my iPod and fired up my iOS Numbers to test this problem. Since the iPod has such a compact user interface, I can only show you a portion of the expression in the editor, but I think you can determine that the function is working as I suggested it would.

I'm sorry that I can't bring myself to exactly duplicate your layout since it grates on me to have Header Rows and Columns and not use them.
I so dislike the iOS Numbers app on the iPhone/iPod platform that I rarely use it. Those times that I do use it, it's with great gnashing of teeth as I fumble with what to me is an awkward interface. Ultimately though, a simple example like this can be demonstrated.
I'd like to help you, but beyond resturcturing the solution so as to not use quoted text within the expression, I can't thing of anything else to suggest. There's not much else to look at.
Jerry