It's been my general observation that in most Aikido dojo the Shomenuchi strike refers to an attack that moves in a downward direction. Meaning, the striking hand (te-gatana portion of hand) would strike Nage on the top of his/her head. I've trained a little in Kendo and from what I've learned, you're right about the difference in attacks. Kendo uses different names for some of the attacks that are practiced in Aikido. I'm not sure why this is, probably to simplify the numerous names for similar attacks. I've visited probably ten or twelve different Aikido dojos over the years (not all the same style) and each one refered to the Shomenuchi strike as a downward strike to the top of Nage's head. Of course, that doesn't make it necessarily correct; perhaps just easier to remember / understand. I don't know, I'm not one to judge.
REK - I agree with your statement on the difference between redirective movements in Karate and Aikido. Timing is everything as they say.