Hello my name is Bob.
Bob just found something that he believes there is something wrong with this.
252 Atk Choice Band Urshifu-Rapid-Strike Surging Strikes (3 hits) vs. 240 HP / 180+ Def Shuckle on a critical hit: 222-270 (92.1 - 112%) -- approx. 43.8% chance to OHKO
252 Atk Choice Band Urshifu-Rapid-Strike Surging Strikes (3 hits) vs. 236 HP / 180+ Def Shuckle on a critical hit: 222-270 (92.5 - 112.5%) -- approx. 81.3% chance to OHKO
As you can see, unless Bob missing something, there is no way 4 EVs can make huge difference in approximate chance to OHKO.
Bob only want to report that, thank you have a good day.
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.