Stat mechanics have not been correctly ported to DotA2, so I'll explain how they work in DotA1, how they work in DotA2, and link to relevant reports to this issue. I made this thread mainly to show how spread these mechanics were in DotA1, and hopefully convince the devs to implement them fully, instead of scripting a few skills.
Changes in the base STR/INT (white numbers) keep the difference between max and current values. That means that if you're at 100/200 and get extra 100 points (of HP or mana) from a base stat, you're at 200/300.
Changes in the bonus STR/INT (green numbers) or HP/mana from items or skills keep the % value. That means that if you're at 100/200 and get extra bonus 100 points (of HP or mana), you're at 150/300.
Interactions working correctly in DotA2:
-Decay is a base strength change (on both dirge and the targets). Because of that, if Dirge is at 1/1900HP and uses decay on one hero, he'll get to 77/1976HP. The enemy hero will lose 76 current and max HP, and the he'll be dealt the damage. This is working correctly in DotA2.
-Morph is base stats. This is working correctly in DotA2.
-Armlet: It adds 25 bonus STR, but then a trigger makes sure it behaves exactly as if it was base STR. This is working correctly in DotA2.
-Death Pact: Behaves like "base" HP when you use it (so you get the same increase in current and maxHP) but then you only lose the Max HP when it ends.This is working correctly in DotA2.
Interactions working incorrectly in DotA2:
-Level up STR/INT is base stats. So if you're at 1/1900hp and level up, and you gain 3 STR per level, you'll be at 58/1957. This doesn't include the "+Stats" skill that you can level up. This is NOT working correctly in DotA2., your HP and mana increase in percentage.
-Whirling Death, Essence shift, Astral Imprisonment, Flesh Heap and Glaives of wisdom all add/subtract base stats. This is NOT working correctly in DotA2.. These skills behave as if they added/removed bonus stats, so they don't have as much impact when the hero is on low mana or low hp. For instace, if you use level4 AI on a hero with 130/4000 mana, the current/max mana will become 126/3870. But they should become 0/3870. Same with Essence shift (or Whirling Death) vs HP/mana pools (Essence Shift should reduce 19hp/13 mana per hit, until the target is at 0 mana/hp). In the case of Flesh Heap and Glaives, it should "heal" HP/Mana to pudge/silencer, but it does not, it increases the current HP/Mana keeping the percentage before the stat gain.
- Morph and Invoker gain base stats in WC3 DotA but bonus stats in DotA2. It's not too relevant.