I've read it more than a few times. Here is the explanation verbatim:
"To make an intelligent decision, we need to know which requires more paint and how much more, how long each will take, and we need some information on the labor costs. Statement (1) gives us information on which requires more paint; however, we still need the actual amount, the number of hours, and the labor costs.
Statement (2) tells us the amount of one paint and the amount of labor; we can find from the question stem the amount of labor needed for the other paint, but we still don't know how much labor costs, or how much of paint B is needed. Using both statement together, we still cannot find the labor costs. Both statements together are insufficient."
Clearly, there is an error. This is the last DS question in the Kaplan GMAT Math WorkBook