Why divide the 256 draftees like they did?

Figure 32 *8 works out. But apparently, they just kept drafting on some of the rounds.

 

http://en.wikipedia.org/wiki/National_Football_League_draft#Compensatory_picks

 

Thanks,
I seem to be getting answers. Even though I might not understand this one, I know that there is a reason,

 

Compensatory picks = lose more players to free agency and unable to replace allows that team more picks I in the draft. That's only the basics as I understand it. How they decide who picks when, is not clear to me.