I'll quote what Krinu told me

Simcraft can only answer the questions you pose it. The biggest mistake people make with it is asking the wrong questions, or not understanding the questions they're asking - which results in the same thing.

In this case, you're asking it to optimize around reducing your TMI (theck-maloree index), which is a mathematical construct intended to show approximately how "spiky" a character's health is over time as a percentage of maximum health. Lower is better. Because TMI is calculated in six-second intervals (it performs a rolling average of your health with a six second window and checks how much each value differs), the more healing you do to yourself, the lower your TMI will be.

So what do the stats do?

Well, Bonus Armor obviously reduces the physical damage you take. As an added bonus, the AP you get increases the absorb effect from Tooth & Claw as well as increasing the healing provided by frenzied regeneration.

Leech is basically "more healing." The reason it's ranked so high is that the conversion percentage on it is REALLY good, especially since the 4x return on rating. However, as a tertiary stat, it's not something you can explicitly gear for. Most people don't bother including it in simulations.

Stamina obviously increases your maximum health, which drops how big a hit is as a percentage of your overall health, thus reducing TMI. It also scales multiplicatively with multistrike.

Multistrike increases your health (as lacerate multistrike ticks and AA multistrikes grant 2% extra maximum HP), which like stamina, decreases the percentage of your total health a given hit will be.

You can go down the stat list like this. Basically, simcraft is doing exactly what you asked it to do - but you might not have been asking for what you wanted.>

If you're not pushing progression mythic bosses while at or under gear level, you're better off optimizing for DPS or DTPS (damage taken per second) rather than TMI.