Hey, you should have bought my book, "The Muvipix Guide to Premiere Elements 12"!
From your question, it sounds like by Crossfade, you mean audio crossfade and not video crossfade, right? They're very different effects.
The position that the transition lands on the clips is based where the program judges them best to fit.
As I show you in my book, every transition -- video or audio -- needs a little extra footage beyond the end of the clip in order to create the transition. In other words, in a one-second transition, you need half a second more of the first clip and half a second more of the second clip in order to create the transitional segment for when both clips appear on-screen or are transitioning from one to another. If you are at the very end of the first clip and the very beginning of the second clip, the program has to create the extra frames by adding a still image to fill in for the footage that has not been providing.
If one clip is trimmed back but the other is not, the program will move the transition over one clip or the other to take advantage of the fact that one clip has extra footage to create the transition. (The illustration in my book makes it clearer than my description.)
In short, if you want to control where the transition falls, you need to trim back half a second or so off the end of the first clip and half a second or so off the beginning of the second. (This is not unique to Premiere Elements, by the way. It's how all video editors work.) Then you can manually set where the transition falls.
BTW, as I also explain in the book, the difference between Constant Gain and Constant Power is negligible. Most people can't hear the difference. Though most audiofiles consider Constant Power to create a more natural transition.