It is a handy calculator which has simple and logical system. It was selected as a "No1. gadgets which changed the world". 
Ordinary Soroban has 24 or 27 columns. Each column consists of 4 lower beads which are worth 1 and 1 upper bead worth 5.  So it is easy to see 6 is made by 5 and 2. ( same as  "7=5+2", "8=5+3", "9=5+4).  Japanese Abacus is made by base 10 system. Maybe you know Chinese abacus and old Japanese abacus have 2 top beads and 5 lower beads. Because money counting system were different those days. The ancient people has made up their abacus base on their culture. As my knowledge, there was Mayan abacus which is made up with base13 system.  It is interesting, isn't it?