@ThePaulBunyanTrophy First I wanted to think it was something about "across the aisle", but then I saw that in the previous picture they actually had five kids wide list, meaning it'd be a strange bus (more like an airplane), but that could happen.
But then I found this, so extra seats is probably what it was all about: http://japan-enkai.com/ja/wp/wp-content/uploads/2015/04/%E3%83%90%E3%82%B9%E5%BA%A7%E5%B8%AD%EF%BC%88%E5%A4%A7%EF%BC%8901.jpg
Link to whole page: http://japan-enkai.com/ja/charterbus/bus-seat-arrangements/
Ooo, I even found an English page for simpletons like me:
http://japan-enkai.com/charterbus/bus-seat-arrangements/
(Auxiliary seats are there described as 'flip-out seats')