Glad you get it, I am going to explain it slightly different for others who see this thread. In this case, if you had no bonuses you would need to roll a 15. You have a +5 from modifiers. So Larian could have left the DC at 15 and add 5 to whatever you roll. So you would need to roll a 10 or higher to pass the 15DC with +5 added. Instead, they subtract the +5 from the DC dropping it to 10 and now you just roll a D20. Either way, rolling a 10 works.