All of that happens because since there was improvement on AI. People tend to see, what they believe is true. But since blobage of AI usually one of the colonisers is knocked down fast and hard. You have to take '75% of cases when' with pinch of salt. As usually it means no hard data, only 'I believe it's what I see' reasoning. And from my experience, one playthrough takes 30 days to complete. So "75% of cases" will be translated: I played two games, Commonwealth failed to form in both, I will make a claim on forum and round it down to 75% otherwise no one will believe me. ;-)
We have more than enough reasons to believe it's true. Not the number necessarily (75% sounds ludicrous), but the fact that Iberia has been hit harder due to certain changes.
One of the most interesting ones was the solution to the "France doesn't move its troops" problem.
Units get stuck because the AI can't handle one-province-crossings if they end immediately so nowadays France doesn't take Navarra and stops but fabricates on all of northern Iberia instead.
That solved the issue with France reducing half their army to mere spectators in any following war, but also made them take more from Iberia.
And many other things. Just because the change isn't visible or a huge new mechanic doesn't mean there's no change at all. They're working on a lot of things and some of them have unexpected effects.
Which is one of the reasons why I really really hope Gnivom doesn't fix the sea tile DoW bug. It has the potential to break the map and someone hinted at him trying to do it anyways.
We've had that issue since 1.16 and unlike France, nobody has ever complained. Most people probably don't even know that bug exists. And it could still be one of the most impactful changes on recent patches.
Just to give another example of something you might not notice, but what might change your game drastically anyways.
