21st seemed kind of like Urasawa going "oh shit, I can't end this properly in the one remaining chapter in this volume, I'll make the 'last chapter' like two volumes long!". Which actually turned out to work pretty well imo.
Also this is pretty much my favourite manga and I'm pretty glad to see that other people read/enjoy it because a lot of anime kids I know claim that it's "too boring didnt reade". Also, if you haven't checked it out,
Pluto (same author) is actually pretty great as well, especially for an adaptation of someone else's work.