Billie's injury was, at first, listed as bone bruising but turned out to be a fracture (which I assume would be minor but still need healing prior to playing). I would then assume that Billie was able to keep up with a lot of his training- though modified- despite not being able to play.
Christensen had a back op which would've restricted him much more than Billie's injury restricted Billie- hence Scott had no qualms about putting Billie into the AFL side and may be a bit reluctant to put Christensen back without a couple more games under his belt- mostly to gain fitness as there is no doubting his talent.
Also, Billie had done the entire pre-season before being injured in one of the pre-season games. In contrast, Bundy first had a hip clean-out around Christmas, and then later his back surgery, so no pre-season fitness base at all.
I think this more than anything will mean they err on the side of caution and give him another run in the VFL this week.