You are seeing designed behaviour.

The Verdana maps to Verdana in the font menu. To use the Verdana Italic, you
select verdana and click the Italic style button or use the  Ctrl i
keyboard shortcut or Format Font and check the Italic box. Similarly, to use
the bold font, select verdana from the font menu and click the bold style
button, sim. for bold italic, ... click both the bold and italic button.

It is possible to have all varieties show up on the Font menu by editing the
font file appropriately using a font editing programme such as FontLab. I do
not recommend it.

Verdana Ref is a font installed by some of the MS reference materials
programs. I do not believe there is an actual font file installed for bold,
italic, or italic bold. If you click bold or italic, the Windows rasteriser
will synthesise a bold, italic, or bolditalic version from the data in the
regular file.

Hope that helps.