This is indeed interesting. In most cases, vocals should come from dead center and other instruments should come from off toward one side. Think about how a concert stage or band is setup. The HTM-200s have wide horizontal dispersion so what you might be experiencing is simply an expanded soundstage compared to a tighter one.
However, there are some factors that can wreak havoc; depending on how close to the side walls and how far apart the speakers are from each other, the HTM-200 can benefit from placing them so their tweeters are on the inside (as opposed to the outside). Also, toeing them in about 5 degrees toward the listening position can help "shrink" the soundstage.