HEW rxn: Why do stabilized ylides give E alkenes and unstabilized gives Z alkenes?

LogicX

1. The problem statement, all variables and given/known data

Topic

2. Relevant equations

http://en.wikipedia.org/wiki/Horner%...mmons_reaction

This is basically a variation of a wittig reaction.

3. The attempt at a solution

I'm trying to understand this concept, because the chemistry gods have decided that it should be stated in every textbook and online source, but never actually explained why.

So, making a trans product is better thermodynamically. My thinking would be that the opposite of the problem statement would be true. If something is stabilized then maybe it would be able to "get away" with forming a cis product instead of trance (sort of a kinetic vs thermodynamic control argument). But the opposite is true. If you are stabilized, you make the more stable product. If you are not stabilized, you make the less stable product.

Why would this be?

Thanks.