When you're using a bitmap font and you choose Italic or Bold from the Style menu, the italic or bold type you see on the screen is a manipulation of the plain bitmap typeface. The plain font itself is ?modified--slanted forward for italic or thickened ?for bold--to create the style you've chosen.
TrueType and PostScript fonts give you separate, specially-designed typefaces, not just manipulations of the plain typeface, for italic, bold, and bold-italic styles. These look even better than the modified bitmaps, and are truer to the original designs.