Yes, they did. It was McDowell's corps plus Shields' division (i.e. four divisions at the time, of which one was sent). The divisions Lincoln and Stanton told McClellan to expect and the ones specified by the men who went ashore to conduct the fact finding expedition. (The report said, almost exactly, "recommend McDowell's corps with Shields' division be sent immediately by water".)Possibly because they didn't have three more divisions they wished to send.
Any other explanation, now that one's provably wrong?