After more research into this (looking for "amazon:", rather than ASIN), I found this thread:
https://www.mobileread.com/forums/sho...d.php?t=140210
And, more specifically, this post by kiwidude:
https://www.mobileread.com/forums/sho...70&postcount=6
It seems that it is ignoring the selections with no ISBN listed, rather than list them as an alternate to the "different" source.
At least, that is what I can surmise... Feel free to correct me.