I know this is a very old post, but in case people are searching for it:
My assumption is the player is creating a bitmap of the 2 items being displayed and using them for the transition. It is much easier on the system to transition between 2 bitmaps than to transition between 2 items that may have multiple layers to them. That is why you see it change just before and just after the transition.
Development Team: I am wondering if the smoothing parameter of creating the bitmap has not been set to "true". If it has, maybe using the drawWithQuality() function would give better results than just the draw() function. At least with the former, you can set the quality level as well as the smoothing level.
Of course, when v6.0 comes out, I believe everything will be on the Starling platform, so they may have taken the smoothing into consideration.