You know something, I've just taken a good look at this.
You tell page gallery you want to work in centimeters and the custom size box tells you you need to enter centimeters but the standard size drop down list is still kept in inches. No wonder it's confusing so the 20x30 needs to be the 7x12 option in the list.
For the cost of this program I'm disappointed they are mixing both centimeters and inches when preferences are set to centimeters. Shame on them!
But at least that now solves the mystery
