How are competitors grouped in judo competition?

I mean how do they decide who to put in what bracket? For example why did the world rank #1 48kg woman Dumitru end up in the same draw as 2007 world champion Ryoko Tani?|||ok ryoko tani rocks!ok so just like don%26#039;t say that bcuz ugh i can%26#039;t believe u!and don%26#039;t b mad bcuz dumitru won gold so just build a bridge and b happy she won gold.and ryoko tani isn%26#039;t so bad!shes great!she won a bronze.and she alredy has 2 gold!so ugh!