Adding columns messes up margins

I have a simple document that I want to add columns to. I have the page specced to 1" side margins.

At a certain point in the document I begin a selection that goes to the end of the document. Then I Insert a Section that has 2 columns. The columns are applied to the existing text. For the first affected page, the margins continue at 1". But for the rest of the document the margins are now 1.25".

I have checked Page and Section properties, and the margins are still set to 1", and indentation is set to 0. So I have spurious indentation happening, and I don’t know where it’s coming from. Most of the paragraphs involved are simple Text body, and there are no graphics.