It is never a good practice to apply a faux bold style to a plain face font. Instead, choose the true bold face of the font.
In the case dealing with this solution, the user had applied the Arial font and clicked on the Bold button in the Measurements palette. In the second line the text had the true Arial Bold font applied to it. What this means is that a faux bold had been applied to the first line of text and the true bold font was applied to the second line of text. When a faux bold is applied to a plain font face and a true bold is applied in another location, two actual fonts are being referenced in the layout and traced in the Font Usage dialog. These will display and print differently. See Knoweldge Base Topic ID 30026-1407, 'Faux Bold Font Alert When Printing or Exporting as PDF,' for details on faux bold effects on both output and in display.