The Fixed Rake is a fixed amount taken from the pot. It's not a per-player amount, but the Stats tries to compute it that way so you can see what kind of impact it has on a per-player basis. What it does NOT do is take into account each player's contributions to the pot and weigh the player's fixed rake amount appropriately. It just spreads the fixed rake evenly among players. So in that sense, it isn't accurate. Then again, it's not a per-player amount. And how a non-per-player amount is computed on a per-player basis is, I think, a matter of opinion.
In your example you've configured a 10% fixed rake with a $20 buy-in and $20 add-on. Let's say you have 10 players, and 5 add-ons were purchased. That would make the pot $300. A 10% fixed rake means $30 would be raked. Each player's Fixed Rake "contribution" would be computed as $30 / 10 players, or $3. That's what you'll see on the Stats tab.
As an example of the other way I described it could have been implemented, let's say one player bought in and purchased 3 of those 5 add-ons. His expenditure would have been $80, which would have been ($80 / $300 = .26666...), or roughly 26.667% of the pot. The fixed rake would still be $30, but his part would have been $30 X 26.667%, or $8. I would argue this is more accurate, but again, it's somewhat a matter of opinion, and unfortunately that's not the way I implemented the Stats computation.