Why does dueling fighting style add such a big bonus?

I’ve created a character in D&D Beyond. He’s a level 4 half-elf fighter. Here’s the character sheet:

Character sheet img 1 Character sheet img 2

The character has a longsword and D&D Beyond has calculated the damage roll on this to be 1d8+12. The +12 bonus seems too big according to my calculations. I’m calculating the roll like this:

  1. The base roll for a longsword is 1d8
  2. I add my strength modifier so that’s +4
  3. I have dueling as my fighting style speciality so that’s +2 because I’m wiedling the longsword with one hand

So I calculate the roll as 1d8+6. Where is D&D Beyond getting the extra +6 bonus from?

If I change the fighting style from dueling to say, archery, the bonus drops to +4, which seems correct. Why does the dueling fighting style add such a huge bonus?