The block preview in the Reordering interface is off.
The block preview when reordering should reflect the actual page layout quite accurately.
Editor:

Reordering:

develop as of 10th of October. Also tested with the latest stable release._Do not alter or remove anything below. The following sections will be managed by moderators only._
Ah, was just about to file this as well.
There were some significant changes to the BlockPreview component in Gutenberg. We now don‘t have to scale down and position elements ourselves anymore I think.


I played around with the css property transform: scale(0.5); to scale the original pages. This results in a perfect preview. However, it would mean we would have to reimplement the reordering.
We probably need to adjust the BlockPreview's viewportWidth prop, as Gutenberg uses that for its internal scale(). Right now we do our own, which is now obsolete though. In other words:
scale()viewportWidth so the preview is correct again