Different distributions do have different probabilities of success. e.g. Rolling 15 or higher on 1d20 has an 30% chance of success vs Rolling 5 or higher on 3d6 has a 9.26% chance of success vs Rolling 5 or higher on 1d100 has a 86% chance of success.

But these aren’t comparing the same thing.

A task that is “difficult to complete” in a 3d6 system, requiring 15 or higher, is equivalent to 92 or higher in a d100 system (or a 19 or higher in a d20 system).

Yes, a task that is “average to complete”, e.g. 51 or higher on d100, will have a higher variance than “difficult to complete”, e.g. 92 or higher on d100.

output d100 >= 51 named “51/d100 success distribution” \ p=0.50, sd=0.50 \
output d100 >= 92 named “92/d100 success distribution” \ p=0.09, sd=0.29 \

But that has nothing to do with the dice rolled, but with it being closer to a 50/50 chance.

Whether a neophyte has a 50% chance of failure vs a trained character having 25% chance of failure is entirely based on how the system defines neophyte vs trained, and nothing to do with the dice used.