You're confusing two different issues - your "pricing" can be accurate to a fraction of a penny, but it does not mean that your hedging (replicating) strategy is. Let's say that options is theoretically worth 0.1 and you sold it for 0.3. You still have not locked in any gain. Market may immediately wise up, and come in agreement with your valuation so you would be able to repurchase your option for 0.1, and lock in your gains, but that is very, very unlikely.
Otherwise you would be hedging, that is attempting to replicate option payoff, and mitigate the risk, by dynamically trading in the underlying. But your trading is discrete (let's say twice per day) and in discrete units ( 100 shares ), and underlying may move discontinuously (jump) . That means that your replicating strategy may result in a loss of more or less than 0.2 of theoretical difference you were hoping for. To summarize, your hedging error is not your pricing error.