Why are reflectance values from 0 to 10,000 instead of 0 to 1?

This is controlled with the Reflectance Scale Factor parameter for ENVI data products. 

By default, GRYFN scales reflectance values by 10,000 so data products can be saved as integers instead of floats, saving 50% disk space while maintaining a high level of precision. Users can then divide observed values by 10,000 to get a reflectance value on a 0 to 1 scale.

If users prefer to see reflectance values from 0 to 1, change the Reflectance Scale Factor to 1 and set the Data Type to 32-bit Float.