It is simple. You bid a certain price; the seller has duplicate item or the original sale not not go through and is offering it to you for the price you had offered to pay. Typically, when the offer is made immediately after the auction, the seller had a duplicate, but when it is made some time (as in days) after the original auction, it is likely that the buyer defaulted.
You can spend a lot of time speculating on the hows and whys, but in the final analysis, it really does not matter, since second chance offers are standard (and, more importantly, protected) eBay transactions. I have purchased a number of items in this manner. If you were willing to pay the price and want the item, then it is a chance for you to get it. If you feel that you can get it later for a better price, then simply pass on the offer. I would take it, but that is just how I operate.