Or, does lucinda sans have such a small difference, that my untrained eye can't see it??
No, Lucinda Sans definitely looks different, and it is showing up in your post for me.
What again is the default font for this board?
Different areas of the forum use different default fonts. But posts use Tahoma, arial, helvetica, and serif in that order. Most people probably have all four on their computers, but the way it works is that the browser looks for Tahoma first, and if it finds it, the browser uses that one. If Tahoma can't be found, then the browser looks for arial next, and so on down to serif.