higher bid is on the cards, page-18

The reason it is not at $.15 is that the bid is non binding. AGL can walk away at any time until the bid becomes firm (see their announcement 2nd July).
If they walk away the S/price will fall.
Some S/holders are selling at $.13 because of the risk of the takeover attempt falling over and the S/price falling back.
It will be AGL doing all the buying at $.13
This is great for AGL. When they eventually offer more than $.15 they will already own many more shares which they bought at $.13
Not me however. It is early days and I am a buyer at $.13 not a seller.