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not a rumour, this is how the chains are balanced, otherwise itd literally be shinies as easy as red gyarados.
each ring of tiles moving away from the player gives the following odds of producing further shakes upon the encounter ending (if encounter is KOed):
- one tile: 53%
- two tiles: 63%
- three tiles: 73%
- four tiles: 83%
bump all of those by 10% if you're catching instead of KOing, meaning catching a pokemon in grass four tiles away gives the 93% of producing more shaking grass
crunching the numbers (a website did it; i can't do maths), you have about a 5.4% chance of a chain making it to 40.