Very interesting thread.
From several standpoints, this is highly counterintuitive.
I'm sure the original assumption for what qualifies as "borrowed" means in your possession for a very limited time; exactly enough for you to make your checks and copy your spells while the book's owner nervously paces.
A spellbook taken as loot off an enemy caster isn't "borrowed" and is in your possession indefinitely and in a non-rules specific amount of time the spells of theirs you have studied are as easy to read as the ones you scribed yourself, and just like spell research in between levels it's just assumed you are going to be studying this book to claim ownership over it.
This could have all been written more clearly.