Very well spotted. I never noticed that before.
Having checked my collection and spares I found that the "flat-top" head is featured on 2004 and 2005.
2001-2003 and 2006 have the "high-top". Looks like Krauss have missed it also or decided it didn't merit a sub-type classification.
I didn't have any year that features the two different styles.
I don't have a 2007 or 2008 (as listed in the catalog) to compare.