If I understand what you're saying correctly, you mean the number listed in the secondary weapon's box in the cockpit right?
In that case, this isn't a bug, it's just how the game groups together the drop tanks. Drop tanks of the same volume are listed together the same way bombs of the same weight are listed together. The number is just telling you the total amount of fuel you have available in those drop tanks. For example, your 2x150 gallon tanks are listed as 301 (I guess they don't hold exactly 150 gallons) and your 2x75 gallon tanks are listed as 150 when full. If you take up an aircraft that can take drop tanks of multiple volumes like the P-47M, you'll see that you'll have multiple drop tanks to cycle through.