I see this, too.  I just colored all messages received from "Def, Abc" and
all messages from that person were colored.  However, I also have messages
from "Ghi, Def" and those, too,  However, messages from "Jkl, Abc" where
not.  That is, when I colored messages from someone whose last name happens
to be someone else's first name, both entries were colored.  I haven't yet
worked out exactly what's happening, but
http://www.howto-outlook.com/howto/coloremailadvanced.htm indicates that the
pattern matching is not simple.