When 'Use Preview Bounds' option is enabled, Illustrator included strokes of the selection into the bounding box.
However, if we try to resize a selection, Ai would make the object suddenly larger, as if the option to respects preview bounds is disabled — take a look at the GIF attached.

If we also enable the 'Scale Strokes & Effects' (not shown), we can somewhat understand, why the team chose to do this — if the preview bounds were used to resize with both options enabled, we would get the selection move slightly to respect the scaling of the stroke... but this feels exactly what one might need with these options enabled.

This needs some thinking over, but the current approach stays confusing.
There are other reported problems with the options...
Here, Scaling proportionally not working with Use Preview Bounds enabled: http://illustrator.uservoice.com/forums/601447/suggestions/48166034