I am sure to be corrected if wrong. 
Synthetic oil is very slippery.
As a clutch wears or becomes older , the pressure between the plate becomes less , therefore resuling in greater slippage over a non synthetic product.
As with new clutches, not all are created equal. Some high performance clutches have more presssure between the plates and are less likely to slip while new , but as that pressure decrease with time slippage with a synthetic would be more noticable.