The basic deal is this: 8 and under have a short playing field and slower speeds. 9 kept the slow speeds but increased the field size, so it feels odd to some players. In 10 and up they kept the field size but increased popkun speed to accommodate, so it feels closer to 8 and below.
So 9's really the only oddball since after 10 they made the speed proportionally accurate, though 8 and under have speed issues because they only had up to x4.
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