If a grain seems extra heavy, you know it's a different kind. Not all types of grains are the same. So when you feel the heavy type, you know it isn't the same as the light kind.
You sort all of the different kinds and types by putting them in the right chutes (しゅ). They slide away, probably to some big type-based piles you can't see.
Currently there are no similar looking kanji which exist in the WaniKani learning system