The difference for me is that on other people's records, the artists want James because they want his voice, so LaBrie always sounds very "in the pocket" and the material usually lends itself very well to the uniqueness of his voice. On Dream Theater albums, LaBrie always sounds like he's being inched out of his comfort zone into singing a style that's not totally "him."
LaBrie has pretty much unlimited studio time, a huge budget, and additional producers to work with when doing Dream Theater records, so of course they always sound more polished. Whereas on other projects, he likely only spends a day or two in the studio recording his parts.
Either way, I still think his performance on The Human Equation is better than any DT record or JLB side project.