One way to look at the die roll: It's what pops into your head.
At DC 20, it's a little something an expert would know. That is, someone with a +10 modifier giving a lecture could take 10 and give you DC 20 information all day.
Someone with a +0 could have heard something cool at one time and it left an impression.
Someone with a -1 cannot make DC 20 (at least not without an item or other bonus).
And that is what it means to have Int 8 in 4e.
You could have been told a thousand things in school that an "expert" would know, and they go zoom over your head.
