I dont know what is up with the different sized group 65's, as it is a designation of dimmensions.
Now for some fun, this changes from time to time, but the last I heard, Sears, Ford, And Deka where all made in the same place, LOL. Now Deka does make three levels of batteries, I know them as 565, 665, 765 for group 65 series. They have differnt CCA's and warranty periods. Might account for the variance in price. Or Ford may have change suppliers.
Here is a quote from an article about CCA sizes "The most logical step would now be to buy the biggest nastiest battery we could fit under the hood. This is not necessarily true. Say your original vehicle came equipped with a 650 CCA rating. Upgrading to an 850 CCA battery should be more than sufficient. Why not skip the 850 and go straight for the 1000 CCA battery? In order for the battery manufacturer to create a 1000 CCA battery, they need to cram more plates into the same amount space as the 850 and 650 CCA batteries. More plates means more power, but it also means less durability and a shorter life span. So if you don’t need it why waste the money on it? You would be better off spending your money buying a better quality battery. This makes a lot of sense, but we had been in the industry 20 years before any manufacturer explained this to us. "
It should also be noted that Deka usually rates their batteries light, so a 770cca battery from Deka would likely test to the same spec as 1000cca Interstate battery.