Column width and document margins don't add up I am sure this question has been asked many times, I just couldn't find an answer on the web.

I am writing a letter (I started with one of the Pages templates).
Now I'd like to increase the width of the body of the text.

So I set the right Document Margin to 0; that increased the width a little, but I get nowhere near the edge of the paper.

I've got a paper size of A4, which has width 21 cm.
Looking at Format / Layout / Columns, I see that the column has a width = 14.5 cm.
Left document margin = 2.5 cm.
But 2.5 + 14.5 = 17 cm, not 21 cm! 

Where is the rest of the width?
Why can't I increase the body text width further?

All hints and insights will be highly appreciated!
Best regards, Gabriel

PS:
Apple Pages version = 10.0 (6748), macOS Catalina (10.15)
At the very top of an open document click on the 2 Document tabs on the right. This brings op options to set hard page boundaries. The first Documents tab I'm referring to is the one next to the Format tab and the next is below that. See image below. 



If that doesn't work add the paper size US Letter borderless available via pulldown.