What do you mean by underprice implied vol? I think what you mean is that vol is underpriced, not implied vol being underpriced. Implied vol is just a function of 4 factors known at at the time of the trade (underlyer spot price, option premium, option strike, time to expiry) and one unknown (risk-free interest rate over the lifetime of the option). Saying they've gotten implied volitility wrong implies they've mis-entimated the risk-free interest rate. I think instead you mean they under-estimate vol, not implied vol.