Obviously because the ones that have a jiggle to them are not wearing a bra??
I not 100% convinced this is the case; first off in real life, breasts still have some movement even in a bra. Having tried out a fair number of the different models in the game, and a number of those have a bounce that is consistant with being in a bra AFAIK (ie theres a bit of bounce across the top to indicate breast movement while "restrained" in the bra). I can't remember all of the ones that fit this, but Therese has some bounce, but is clearly supposed to be wearing a bra and one of the Chinatown models as well.
This is also born out by looking at the odd way certain models deform under the "money x" code. The ones that don't change at all under this code are the breasts that don't move at all. Amid those that do respond to the money x code, the level of "bounce" is different as well; the malkavian and Rosa has a bit of "swing" and bounce, some (like one of the Chinatown models I mentioned earlier) has just a minor bit of bounce, one of the "stripper" models rolls around like a pair of loose cumquats and so on.
So it raises the question why some are bouncy and others aren't.