I assume that your project panel shows your BD Transcode Status as "Do Not Transcode."
Everything you add to a disk uses some space. I generally start with the belief that disks that do not appear to hold as much as they should are a problem of how Encore reports the sizes. In the project panel, Encore reports GB, as you might see in Windows explorer. But in the build panel, Encore puts GB after it, but is reporting bytes. See my comments in this thread.
http://forums.adobe.com/message/4838089#4838089
I don't know how this applies to Encore's calculations/comments about bluray.
I do not know how it applies to the current version, but Encore has padded Bluray estimates (making it hard to fill the disk) and in some cases, broken up files during the build and potentially added size.