Chain stretch and lower gears worn will contribute to the overall length of the chain. When the gears are worn the chain will sit lower in the groove, a .5mm here .5mm there will add up. Since the R/S cam is the last gear it only stands to reason that timing mark would be the most effected. As well if the chain sets deeper in a rail in the lower end of the engine, the semi curved one, it too can add to the length of the chain. If your engine has 250k or > on the original lower gears and rails it may be a good time to consider going in! New chain, all gears, all rails in the chain drive assembly could reduce the majority of the offset, may not get all of it, but the offset keyways will assist you with getting the timing on the money.