The starting expression "64x^3+125y^3" can only be factored into the two factors "(4x+5y)(16x^2-20xy+25y^2)".  If I examine the second factor and decide that something like "(4x-5y)(-5y+4x)" should be a further simplification, then second factor is not equal, giving "16x^2-40xy+25y^2" (...close, but not correct).