Quality is not being lost during export to Blu-Ray disc. Popular HD camcorders produce files of about 25mbps maximum, and Studio’s Blu-Ray disc exporter supports 40mbps (at Best Quality). The 63% designation refers to the difference between the source files on the timeline and the Blu-Ray export Best Quality setting.
Here are some examples of bit rates of video types.
HDV = 25mbps
JVC Everio HD = 26.6mbps average
AVCHD = maximum of 24mbps, depending on type; consumer cameras may be closer to 17mbps.
Because Studio supports 40mbps for Blu-Ray, the only way to display 100% quality in the discometer for this format, is to use a source format that is actually 40mbps.