Ever try to vertically center text in a div and it's just not working correctly on the Mac? But it's spot-on correct on the PC? Or vice-versa?
You've tried everything: the line-height set to font size, or 1px lower. You've vertical-align'ed: center the hell out of evertything in your CSS file. No matter what you try, nothing seems to make a difference.
In desperation you switch from your beautiful (expensive, too!) custom font and fall back to plain old Arial as a last-ditch effort to get something to look right. And BINGO it's now perfect.
Well, it just might be the font file's fault. Evidently the font designer needs to account for something called Vertical Font Metrics. So, depending on what metrics they use to determine such things, you can wind up with odd display behavior between browsers and OSes.
Now, about this bold/bold thing....