Can someone help me understand this?

[Cehrabehra]

Ok I have one guess: Lower half length. the higher that number is the more fire and flashes the diamond has. Both diamonds are listed as 80%, however GIA rounds the actual number to the nearest 5%. Example,both a 77 and 82 measurements will be reported as 80% lower half measurement. If hers is higher it might produce more sparkles and contribute to a whiter appearance. Check out this video from goodoldgold, watch in high resolution.
http://www.youtube.com/watch?v=-9FtRj_IG9Y

I really don't think this has anything to do with it... I've seen both types perform very, very well. cut the same one wouldn't appear brighter, but may have more frequent, yet smaller, scintillation. The other bigger, less frequent. Both should look equally brilliant overall.