sMON represents a share of the total pool of staked MON. The staked MON earns yield over time in the form of more MON, so the amount of MON in the pool grows relative to the amount of sMON outstanding. As this happens, each sMON token represents more MON in the pool — holding sMON gives you both your initial stake and your share of the yield earned since. This ratio never resets to 1:1, so the value of sMON relative to MON grows continuously over time.