I just wrote about this elsewhere, but …
Each tile is assigned to a specific hero. You can see this if you assign different troop types to each stacked hero. The associated hero determines which talent (if any) has a chance to trigger when the tile hits.
For example, you stack Barbarian GM, Sorcerer Natalya and Rogue Marjana. A red match-3 happens to assign one tile each to these three. GM’s tile may Wound, Natalya’s tile may Delay, but Marjana’s tile can’t trigger any effect (rogue’s dodge only applies to incoming damage). If you leave Marjana out, running only two red heroes, then the odds of triggering a class ability go up from ⅓ to ½.
This design appears to be a subtle nerf to color-stacking.