I don't know. IE clearly gets the key event. Maybe it's not forwarding it on to the currently loaded page? Seriously, I don't know.
Web pages (and the TD, which is related), can register for key events on different parts of the page. Whether or not the press of an actual key results in an event depends on which part of the page has focus. Events filter "up", so parts of the page that contain other parts of the page will also "receive" those events. For example, take an HTML <table> that contains a button. When the button is clicked (other event types work the same way as key events), the button will receive an event, and so will the table in which it is contained. The TD listens for events at the top-most level, so that key events that happen anywhere on the page will be received. So, while I guess it's possible that "focus" has something to do with it, it shouldn't since the TD should get all events. Or, as I mentioned before, perhaps only the "key pressed" event is being generated by "SendKeys", but the TD relies on the "key down" event.