The camera does not show a full sheet of Proofgrade material on any Glowforge. You won’t see the top or left edges.
Your work area is approximately 11"x19.5" while the wood is 12x20".
If you look at where the print head sits in its home position, all the way back and left of the case, it can’t reach the top 1" of your wood which goes further back than that. The camera reflects that.
There’s actually another 6" of space beyond where the print head can go because the laser tube is in the way. You can fit up to an 18" deep sheet into the machine but won’t be able to see more than the bottom 11" of it.
To avoid printing past the bottom of your sheet, push it forward enough to completely cover the honeycomb area of the tray and that black line under it.