Stats are calculated based on the Pokemon ID of the pokemon.
If the Pokemon ID is divisible by a certain number X, it is given the +Atk stat. If it is divisible by Y or Z, the associated stats +Def and +Spe are given.
It is not calculated. The very first pokemon in the game had the ID of #1. Each pokemon obtained thereafter has the next number as their ID.
That means if your Pokemon's ID is #150269610, that means it is the 150,269,610th pokemon obtained in this game. It's that simple.
Here 'obtained' can mean caught, chosen as a starter, received from events and raffles.
This means the rate of pokemon with stats is permanent. It cannot be changed. Because if that X in the above divisibility test is changed, all pokemon that have the +Atk stat will lose that stat and some other pokemon will receive that stat. And Pokemon losing their stats is not desirable.
This permanence also means the rate is same for everyone, all the time. And that applies even to pokemon obtained in the game even before stats were added
Yes. The X, Y and Z numbers cannot be the same... because maths. Otherwise you would not get Pokemon with more than one stat.
While I will not be sharing the values of X, Y and Z, it is very easy to figure out with a basic calculator. Once you know the values, stats of any Pokemon can be independently verified.
You haven't been paying attention to what you've been reading on this page.
No, it's silly.
Set your own goals; goals that make you happy.
Don't collect pokemon with stats just because other players collect them.