Is this volume difference normal in practice 4.14

Hi again, I'm still stuck on practice problem 4.14. I've redone this exercise several times (supposedly according to the PDF, 20 minutes is enough, but I've been at it for over an hour) and I always get the same result: Volume = 992.53 cubic millimeters, but according to the PDF it should be 993. Is this normal? Could it be a tangent issue or something?

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