I think a success rate % would have been even more informative - its seems fairly obvious that the players with the most chance of success are found in the 1st round.
Positions regarded as 'high impact' - such as CB, DE and OLB will always be more heavily selected in the earlier rounds.
I think this comes down to the fact that their stats are always alot more accessible (INT's, sacks) and its easier to compile highlight reels of their 'big plays'.
Safeties are gradually coming back into vogue, just like TE's did a few years back - I read an article where these guys were often CB's who were relatively substandard athletes, but now you see 3 or 4 of them with 1st round rankings.
Even though its not mentioned (with typical anti-Patriot bias
) all 3 of New Englands D-line starters are 1st rounders.