The conversion market is accurate using 3.90 and .66 for last sale on the 27.50 strike for March. Perhaps you're valuing the calls with a higher risk-free rate.
Since C is listed on 6 exchanges and has many mm's at all 6 places making prices I am inclined to go with their prices over your theoretical value calculation.
Do you really think that with the single volatility value that you put in or is there as default you are gonna get theoretical values that the market is pricing in!?