I've noticed this too, (though didn't check details as you did). I think the reason is either:
1)because one is a Mage spell and the other a Cleric spell. The Cleric cannot learn it but only a Cleric can use it, in this case.
2) one is simply a scroll spell that anyone can use and cannot be scribed into spell books.
I do not KNOW this - only come to this conclusion through observation.
Hope this helps.
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