This might be what is happening: Each paragraph in a Notebook has a ruler associated with it. Choose Ruler from the View menu to see the margins that are set for the current paragraph.

The default ruler in a Notebook has a flexible right margin. That is, as you resize the window, the right margin automatically aligns with the right side of the window.

If you make a manual change to a ruler, then its settings become fixed and do not change dynamically when the window changes. The ruler for the cells you see a fixed margin for are probably of this nature. You can set that by hand, but in your case you copied and pasted some formatted text (likely RTF or Word format text) where the ruler information is included on the clipboard.

The easiest way to remove the formatting is to select the paragraph and use the Plain command from the Font submenu of the Format menu.

You can also use the Copy Ruler and Paste Ruler command from the Copy and Paste submenus of the Edit menu to do this as well.