Answered my own question. The input from Pig is a bag of tuples. In my case each tuple has one element, e.g.: {(-0.01), (-0.03), (0.00001), (-0.2383), (0.158)}.
So in order to compare it to a float-type element from another list bins, I need to insert something like:
delta = list(delta)[0]
between lines 16 & 17 above, to pull out the float-typed data element that is the content of the tuple. Then the comparison on line 18 will work.