The gemara concludes that if one steals an item and ruins it, he is not able to simply return the broken item and pay for the damage, rather he has to pay in cash for the entire item, or replace it with an equivalent item. However, when one damages, or borrows an item and damages it b'ones, he can simply return the item and pay the depreciation amount. Why? Tosafos explains that when one steals an item they immediately acquire the item by removing it from the reshus of it's owner, and therefore are liable to reimburse the owner for the entire item (not just the difference from the time it was stolen and the time it is returned). But, when one damages, he is only responsible for the amount that the item depreciated due to the damage, but whatever remains still belongs to the original owner. Based on this, a borrower who is responsible for o'nes is also considered to acquire the object when he borrows it and therefore is responsible for the entire item.
Why is a "sho'el", a borrower, considered to make a kinyan at the time he accepts shemira? Just as a shomer sachar is only responsible for what was stolen but can return whatever remains and just pay the difference, a sho'el should be able to do the same? Tosafos understands that since a sho'el is responsible for o'nes, he responsibility cannot begin at the time that the o'nes occurs because one cannot be liable for a complete accident. The only way that a sho'el can be responsible for o'nes is because he makes a kinyan on the object when he borrows it. Based on this, there is a major difference between the chiyuv of a shomer sachar/shomer chinum and a sho'el. A shomer sachar/chinum is responsible for their negligence in not protecting the object, and that obligation begins at the time of the incident. A sho'el on the other hand, is not responsible for the incident but responsible at the moment he borrows to return the item as it is at that moment.