change the number of posts required to change level.

At first glance, it seems that the changes are significant and there's a shift of one level, but this is just an illusion, and if you look closer... using the 3rd level as an example:
* Level 2 - >= 25 posts
* Level 3 - >= 75 posts
* Level 4 - >= 150 posts
So, to reach level 3, you need to have more than 74 posts, but less than 150.
[75 <= Level 3 < 150]

* Level 2 - < 75 posts
* Level 3 - < 150 posts
* Level 4 - < 250 posts
So, to reach level 3, you need to have more than 75 posts, but less than 150
[75 < Level 3 < 150]

TOTAL:

1. [75 <= Level 3 < 150]
   [75 < Level 3 < 150]
   The shift wasn't by one level, but by one post :)

But I'm interested in another question: why was this particular example chosen for the explanation? Only in this example is there a division into ranks by 1 post. Let's take something else as an example, say, the 6th rank, and see:
Before:
* Level 5 - >= 250 posts
* Level 6 - >= 500 posts
* Level 7 - >= 750 posts
So, to reach level 6, you need to have more than 250 posts, but less than 750.
[250 <= Level 6 < 750]

Now:
* Level 5 - < 500 posts
* Level 6 - < 750 posts
* Level 7 - < 1000 posts
So, to reach level 6, you need to have more than 500 posts, but less than 1000
[500 < Level 6 < 1000]

Total:
[250 <= Level 6 < 750]
[500 < Level 6 < 1000]

And in this example, the difference is significant - 250 posts, but not 1. The approach to considering this situation can be different; I personally am an optimist, but is this rational?