Bill Zebub
“It’s probably Matt Mercer’s fault.”
You are adding a +1 to a D20 roll. Obviously if you need a result of 20 to succeed and you were a +0, a +1 will double your chances of success. But you are also only increasing the result of the number you end up with after adding multipliers by 5%.
As far as what was responding to, that may well be part of the issue. You keep repeating things like "it's already been explained" or referencing a previous post without linking to anything or explaining "what has already been explained ". So if I'm confused, I apologize but perhaps something a little more enlightening than "has nothing to do with " might help. You may have a clear understanding, but obviously other people don't.
1) You said “increase effectiveness by 5%” and that is false. If you are getting a 5% return, an investment that returns 6% is 20% better, not 1%.
2) You were responding to a post in which I was agreeing that the +3 vs +2 thing is about comparing your own character options, not comparing your character to other characters. If I were playing a duet game (just me and a DM) I would find the 16 no more nor less desireable.