Printing in LR4 - Cell Size Doesn't Match Print

I aim to print an A5 greeting card on scored A4 paper so that my image fills one half of the page, borderless. So I select:

1) Custom Package

2) Set paper size to A4 (210mm x 297mm)

3) Cell Size: Width = 210mm, height = 148.5mm

Can anybody tell me why the page shows the image filling less than half the page? This does not make any sense? The cell size is shown as 210 x 148 but the scale shows the image as smaller? This leaves a white margin around the image on two sides.