Various news organizations are reporting about Trump not being fully briefed about the order, among them Fox News, The Independend, or the New York Mag.
The original source for the claim seems to be the New York Times:
> [F]or the moment, Mr. Bannon remains the president’s dominant adviser, despite Mr. Trump’s anger that he was not fully briefed on details of the executive order he signed giving his chief strategist a seat on the National Security Council, a greater source of frustration to the president than the fallout from the travel ban.
The report seems to be based on insider information, and obviously doesn't name those sources.