There is more length in the outside ring.
For a sensor of any given resolution the wider the disk the more lines that can be scribed on it.
While in a way this relates to frequency, it is more about sensor resolution.
just curious, does your optical encoder use Gray code? it should. if it does, the two LSBs (of Gray code) will have the same "frequency" but will be 90o outa phase. just like the encoders of the earlier mice.