My guess is you have the horizontal/vertical alignment not set to "center". See below.
I've been fighting this issue for a while. I've managed to fix it on some workflows, but I don't understand HOW I've fixed it, and it takes too long to iterate various impo settings and re-process the job.
Imposing 2 8.5x11 on a sheet. The files have a .125 bleed, but I'm only using a .125 gutter. The gutter will contain ALL of the bleed from one piece and none of the bleed from the other.
What am I missing? Where would this behavior be changed in settings?
-Dave