Illstrator preview ground transparent vs white

This is more of a "huh, I wonder why this is happening" curiosity thing.  When previewing thumbnails in Bridge some of my Illustrator images show a transparent backfound and some are white.  Although I attached two images, there are 4 in the series.  The first image was created, then "save as" for the second and the second "save as' for the 3, etc. Two of the images preview as white ground (2 and 3), two preview with transparent ground (1 & 4).  I cannot see any settings, appearance, transparency, document info, etc that is different from one to the other.  All pieces have the same sourced art files. 

Any ideas why this happens? And will curiosity kill the cat; the cat being me? thanks.