As an old highly-ranked competitive Pokemon player, I fully endorse Hunter Island’s system; IV’s and EV’s are a pain.
To clarify- Agent, that’s not how it works. Mostly. Pokemon differ *mostly* due to innate IV’s, randomly generated for each Pokemon you encounter. They cause a variation of +/- 32 for each stat at max level (so a Pikachu could have, say, 268-332 Speed at level 100 based on its IV’s).
What you’re talking about are EV’s; every time you defeat a Pokemon in battle, your Pokemon gains EV’s based on the opponent. Killing a Zubat, for example, gives 1 Speed EV. Killing a Charizard gives 3 Special Attack EV’s. You can have a maximum of 252 EV’s in one stat, and 512 EV’s total. Every 4 EV’s in a stat gives +1 at level 100. Killing lots of small enemies builds up your EV’s faster- but by the time you get to 100, you’ll have maxed EV’s no matter what, so whether you killed lots of little guys or some big opponents is inconsequential.
The more important point is that when you combine EV’s and IV’s, there’s enormous variation in stats. Let’s take the Pikachu- given 252 Speed EV’s could supply up to +63 speed, the variation in speed for a level 100 Pikachu is 268-395. That’s huge. And that’s not even counting NATURE, which can influence things even MORE dramatically.
Of course, when you’re doing competitive battling, everyone uses Pokemon with all max IV’s. So the practical range is smaller.