A greater percentage of the playerbase have cleared DS NOT IS EQUAL TO Cata has more raiders.
That's all I personally want you to understand, but I feel like I'm explaining poetry to a 5 year old.
And just so we get this straight.
20% DS include LFR, normal mode and Heroic.
Karazhan had one mode and it was NOT as easy as LFR, and quite frankly a little bit above DS normal mode.
LFR = falling asleep
Karazhan = be on the ball.
Due to this difference in difficulty, you cannot compare statistics from that period with the current one. 5% sunwell means some people were really fucking good. 20% DS simply means a lot of people are capable of sleeping with their eyes open.