Seem similar, do they? But they have a *radical* difference, one is "old" (X-KDE-Keywords) and the other is "new" (Keywords). The "new" one is also an xdg standard and as such the separator is ';'. The old one is just a KConfig string list and thus the separator is ','.
Great isn't it?
X-KDE-Keywords uses , for separation
Keywords uses ; for separation