I don't know, but the only reason I see that makes sense to me is that the blades on the 990 and the terminals in the power cord don't match the IEC spec for size and/or spacing. I've seen cases where even a slight mismatch between the bade and the terminal creates a bad conection that heats up enough to cause a fire. (Thermal Event for those who don't like the f-word)

I'm not saying that WILL happen using an IEC cord with the 990, but it's possible.