Suanpan Chinese Abacus Question

Hello, wondering if anyone could advise as to why this Suanpan is different to most? They normally have rows of 5 beads on the bottom layer and rows of 2 on the top. However, there are two rows on this one which are different a 4 and a 6. The piece doesn't come apart so it doesn't seem possible that this was done after production and wondered if it was just a mistake or has a purpose? Thank you...

I had one of these as a kid (many moons ago ). I think it’s a simpler version for children to learn on. Is it possible one of the beads split and it was glued back together on the wrong stick?

Thank you Naomi :) I've seen lots of this type online but they all have 5 and 2 rows.