setup and hold uncertainty
I think it has the relationship with the nature of setup/hold check.
Let's take a simple example, maybe you set the uncertainty include the clock skew and cycle to cycle jitter, for setup check , the tool will check the data arrival time in the next cycle cycle.
But for Hold check ,tool will check the data arrival time in the current cycle ,so ,there is no reason to set the hold uncertainty include the cycle to cycle jitter,becasue it will worse the timing result .
That's why hold uncertainty always less than setup uncertainty.