It's a safety feature AFAIK. The industry standard for RAM, JDEC, calls for a maximum ram voltage of 1.8v. The newer, faster RAM can't work with such little voltage at it's listed specs BUT to ensure someone building a new system can still boot, there's a feature that lets the RAM boot at slower speeds if only a single stick is detected.

I could be completely wrong and it could be something to do with electrical physics (the resistance of one stick being less than 2), but I know for a fact that other people with similar RAM types have had a similar problem and it's been solved by upping the RAM voltage to the manufacturers spec (on the packaging usually)