Shares of GW Pharmaceuticals (NASDAQ: GWPH) fell 13.3% in June, according to data provided by S&P Global Market Intelligence, despite the biotech gaining Food and Drug Administration approval for its epilepsy drug Epidiolex.For the most part, the stock market values companies based on expectations. Biotech companies often shoot up after positive binary events, such as FDA approvals or clinical trial results, because there's uncertainty about which way the event will go.