Creatures on Vissidal have a loot profile that is modifed by a "Quality Flag," which influences the liklihood a stat will be on the higher end of the possible range of values. Because several stats need to be in the high range for an item to be considered really good, the principles of combinatorics work to make not just more likely, but far more likely to find a good item on Vissidal than any other place currently.
Django verifies this on Turbine's Forums in this thread:
For the curious, here's an example of how this works (all values are hypothetical, the precise values the quality flag uses are unknown).
Remember the olden days when only the damage of a sword was important? Say the max damage can range from 8 to 16. That means 1 in 8 swords will be max, or 12.5%.
What if you also want a sword that has a max attack value, and it can range from 1 to 5. Now you need a sword that is max damage (1 in 8) and max attack (1 in 5). Which now puts max swords at 2.5% of all swords that drop.
Add in melee def max that ranges from 1 to 5 and you drop to 0.5% (half a percent), or 1 in every 200 swords that drop.
As you can see, for each characteristic you add, like spells, workmanship, particular type (like tachi), you dramatically reduce the odds. And I used very small ranges, these days the ranges are bigger.
The Quality Flag also reduces the maximum worksmanship of items in the tier by 2. For example, in Wealth 6, the worksmanship range is 4-10. In Wealth 6 (Quality), the range is 4-8.