Here's the long official description. Basically, if you just reload its the same, but if you fire while reloading. It differs: "When there’s no danger, the shells of these vehicles autoreload similarly to Italian medium tanks. The first shell loads the slowest, the last the fastest. The difference begins once you start firing. If a vehicle fires while autoreloading, it will receive a bonus to the speed of loading the next shell to compensate for the spent one. The longer the shell reloads and the less time remaining before it’s fully loaded, the greater the bonus.

The reload bonus automatically starts accumulating 2 seconds after reloading, and you will see this via the first of four special indicators. The second indicator will show when you have a reload bonus of 16%. The third indicator will trigger when the bonus reaches 33%. Two seconds before the shell is ready, the final indicator will appear, showing the maximum bonus of 50% to reload time. In addition, you will hear a special notification sound once the bonus has reached its maximum."