There a few factors at play here. First, let me address the difference between the 409A valuation and the recent round of financing. Venture capital investors typically invest in preferred shares of a company rather than the common shares issued to founders and employees. The preferred shares can include special rights and protections, such as: (1) liquidation preference, (2) dividends, (3) conversion features, (4) participation rights, (5) ratchet provisions, and (6) board sets, etc.
The purpose of the 409A valuation is to quantify the valuation of the common stock for tax and financial reporting (stock compensation expense) purposes. When there has been a recent arms-length market transaction (like a new round of financing), the 409A valuation is typically based on the "backsolve" method, in which the price of the common shares is inferred using the price of the preferred shares, the right/characteristics of those shares, and other assumptions (including expected volatility of the company's equity, term until a liquidation event, risk-free rate, and discounts for lack of marketability). Thus, the value of the common shares is often significantly lower (20-50% of the value of the preferred) than the valuation of the new round.
Also, the difference between your option exercise price (X) and the 409A price (X + $0.50) reflects the growth in the value of the company since when you were first granted those options. Typically, the options are granted at the 409A price (there are tax issues if the grants are below that price), so the 409A price at the time of your original grant was probably X.