Highlighting. Bar cursor vs circle cursor.

I know that the standard highlight function will not highlight a scanned document.  Sometimes when I click the same highlight tool that I've pinned to the toolbar, I get a small round circle instead of the bar.  The circle is like a fat marker, and I'm able to highlight freehand, anywhere on the page even if is a scanned document, and I have NOT run the text recognition.  I can't find answer as to why it will do that this time but not next time.  It seems random.