If you use a design canvas of 20" x 12", the design will always fall on the same spot in relation to the bed. Given the actual accessible area for the laser on the bed, you will never be able cut at a 0,0 on the preview of the work area.
This question has had a history of robust discussions. In fact, of the many topics that have had lots of energy, I think there were some flags thrown in some of the topics. Take the following thread for example:
Understand that this was at a time shortly before the first pre-release units when into the wild and all the forum had to chew on were pictures posted by beta testers (we couldn’t ask them certain questions) or what Glowforge itself demonstrated and @dan shared.