There may more than one path through the combinational logic between the two FFs. We consider the maximum delay path between the two FFs for setup and minimum delay path for hold violations. So, if we increase the positive skew, we can avoid the setup violation with respect to the maximum delay path. But this may affect the hold requirement of the minimum delay path if the skew is very large or if the data required time to avoid setup and hold violation are very closed to each other.