1 + 1 = 2 _ increase of 1
2 + 1 = 3 _ increase of 1
3 × 3 = 9 _ 3 times more
You need both modifiers to reach the max potential because 3 times 1 is only three, but by have two increase by 1 you end up with 9!
That is how the increased and more work.
The crazy part is when you do X fire damage which critical hits for y% more damage with z increased damage and a multipled damage. Also you have b % more burning damage and increased burning damage and ailment % increased damage and so forth. ;)