Verdana shows up as Verdana in the far right cell, the other two as Arial (I believe).
Now, there are a couple of things to consider when it comes to fonts on Unixes. First - XWindows handles the fonts. The underlying OS doesn't handle fonts at all, and nowadays XFree86 is the most common incarnation of XWindows, even on several commercial Unices.
Rendering anti-aliased fonts are only done with TTF fonts I believe. So if you check what fonts are available to, say, a GTK2 application, you might notice that some fonts that you know are available on the system isn't available to the application you're using. I think this is what's happening.
I don't have a truetype Helvetica font, so in the first cell my browser ("Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.6a) Gecko/20030929 Firebird/0.7+ "if you're curious like web developers are
) proceeds to the next entry which is Arial. Next cell is Arial, so Arial is displayed there. Third cell is Verdana, which also is installed as a TT font, so it's available and gets correctly displayed.
Btw, font sizes can vary wildly between different platforms when it comes to rendering web pages, so I suggest you use CSS more.