Well, the seals around the valves seperate the valvetrain (which is obviously heavily oiled when the engine is on) from the intake/exhaust ports and from the cylinders themselves. If the seals are leaking, the oil that settles in the bottom of the valve indents in the top of the head would leak down along the valves into these areas.

 

so there's no actual seal, like an oil seal on the cam or crank, this would be a reason for having the valve seats reground instead, if i get you right?