Oh wow it is become clear and easy to understand!Well at 236 EVs , Shuckle has 240 HP, and at 240 EVs, Shuckle has 241.
And here is the Possible damage amounts (for each individual hit).
Possible damage amounts: (74, 74, 78, 78, 78, 80, 80, 80, 80, 84, 84, 84, 86, 86, 86, 90)
Since 240 divided by 3 is exactly 80. For Surging Strikes to not KO that Shuckle, it will need a combinations 74/74/(any number), 74/78/78, 78/78/78, 78/78/80, 78/80/80, 74/80/84, and 74/78/86.
Meanwhile, 241 does not divide evenly by 3. This greatly increases the chances of failing to KO since the triple 80s no longer KOs and along with other combinations that KOd at exactly 240.
I can’t do out the entire math as I should be grading papers, but hopefully you’ll get the idea.
Thank you for your thoughtfulness, Bob really appreciate it.
Good luck with your papers grading!