The design of the cam lobe is different. Solid lifter cams have a long, relatively slow area of lift called an "acceleration ramp" that stacks all the parts together (lifter, pushrod, rocker arm and valve stem) before it begins to lift the valve off the seat. The acceleration ramp height just happens to be the same height as the clearance. (.012", etc.) At the other end of the lobe, the same thing happens, a slow, steady deceleration just before the valve contacts the seat, then another ramp to establish the clearance.
If you put hydraulics on a solid shaft, you get a radically longer duration, but no real performance benefit, since the lift is so slight on the ramps. Hydraulic cams have short, fast ramps, because they don't need to take up clearances before lifting the valve.
Jerry
