The team working on mechanics and champion design is different to the art team. It's not like...
Jeez. Alright, it isn't like:
Riot currently has 100% resources
Devote 30% resources to X.
Devote 15% resources to Y.
Etc.
It's more like:
Riot teams operational:
- Design
- Champion Design
- Item Design
- Art
- Animation
- Texture Art
- Modeling
Etc.
Different teams, different priorities, different people, different team leaders, different lots of things. They can't just say "we're going to do this, and it'll be ready by this time", because game development doesn't work like a progress bar on a research in an RTS game.