然而，粗体字体的border-bottom会比普通字体略低。

没有。在链接 Like 之下，边界距第一个字符 L 小于7px，链接 Pro-Life vs Pro-Birth 边界在字符 P 下面再7px。也许你会被 Light 中的 g 迷惑。

On this page (http://nate.fm/articles) I have two different articles listed. One is bold and one is regular. They are both links with a 3px border-bottom. However, the bold font's border-bottom is pushed down slightly lower than the regular font.

Can anyone tell me where that extra space is being defined in the CSS?

However, the bold font's border-bottom is pushed down slightly lower than the regular font.

No. Under the link Like the border is 7px below the character the first character L and under the link Pro-Life vs Pro-Birth the border is again 7px below the character P. Perhaps you are getting deluded by the g in Light.

P.S: I checked the distances by printing screen and pasting to photoshop.