Higher binning on the GPU chip and higher factory clock settings. TOPs can also usually be OCed a bit higher, but mileage varies on anything IC.
The Giga 660 Ti can be had for $285 w/ MIR thru Sept and it includes Borderlands 2.
Amazon still lists the 670 TOP, though is waiting to get more in. Newegg is quick to discontinue items they have trouble getting large quantities of. It's simply an overhead decision to avoid lots of orders they can't fill and streamlines their database.
The question is, do you REALLY want to spend 50% more for a 670 TOP, and wait to get it, for only 10% better performance? $430 is a lot to spend when you only get marginally better performance.