If I'm not mistaken, its the way NS 4.7 renders HTML. I've designed sites that looked great in IE, iCab, and NS 6, but looked horrible in 4.7.
If it looks good in 6, then you're doing it right. NS 6 just got thumbs up from the W3C on its unrivaled support of standards. In contrast, 4.7 is notorius for not supporting standards properly.
With 4.7 having a shrinking marketshare and a dim development future, in view of NS 6, I wouldn't sweat it too much.
Hope it helps