The amount of damage reduced by expertise is less than the damage increase of the same number.

That's because it is necessary to have it be so in order for equal expertise ratings to balance out due to math.

I will try to illustrate it with some round numbers to make the concept simple.

First, you need to understand how expertise factors into combat. When player A attacks player B, it does this little formula:

Damage * (PlayerA Increase) * (PlayerB Reduction).

Now the increase and reduction are percentages. So, if player A has a 25% damage boost, and player B has a 20% reduction, the formula looks like this:

Damage * 1.25 * .8

Now if you think about it for a second, you'll realize that if we take a number and use exactly the same percentage to increase and decrease it, it won't wind up back at its original value, because once we've increased it, the same percentage of the new, higher number will be

*larger* than the original increase.

For example, if we increase 100 by 25%, we get 125. Now, if we decrease 125 by 25%, we won't wind up back at 100 again, because 25% of 125 is more than 25% of 100. So,

100 *1.25 * .75 = 93.75

See? It doesn't go back to 100 because when we increase it, we are increasing it by 25, but when we decrease it, 25% of 125 is actually not 25 - it's 31.25.

The long and the short of it is that if we want equal values of expertise to balance out perfectly, then we need our increase and reduction values to be such that they meet this formula:

1 = (1+Increase/100) * (1-Reduction/100)

Thus, for example, 1284 expertise gives an damage boost of 23.85% and a reduction of 19.26%. If we use them to modify some damage number, say, 1000, we'll see that in the end, we end up back at 1000.