The way that Yorick's ult makes the clone expire (if it hasn't died) is that when the duration ends, the ultimate deals 100K true damage to the clone; this is counted in Yorick's damage numbers.
There is apparently a bug when Yoricks ult dies that attributes the health remaining on the ult as damage that was dealt by Yorick, also yorick isn't really OP, he just has a strong laning phase, always has, he doesn't tend to hard carry very often though because of how easily he can be kited.