for me a calculation like "x+10%" is a very easy thing.
Yep, it's easy. Let's say you're getting 100 per second of something and you get a 10% bonus. Then it becomes 110 per second.
Now you get another 10% bonus, meaning your total bonus is upped to 20%. Now you're getting 120. Easy, right?
Well let's say that you didn't know about the first 10% bonus. You were looking only at the second one by itself. You might be surprised your income went from 110 to 120, as that's not quite 10%. This is because bonuses are added together, not multiplied.