I have long scoffed at the notion that white sharks test bit humans and once they realized we weren't seals simply lost interest and swam away. I even theorized that it was more likely a bite and wait until the prey bleeds out strategy. Neither God nor the universe creates 2 ton fish by having them pass up free meals! A couple of specials I've seen lately have finally come out and proven me right , at least in some cases.
When great whites attack fur seals in shark invested waters you see the famous bass like top water strikes and relentless attack. That's because if they don't kill the animal and munch down immediately another great white will steal there dinner.
However, when they hunt young elephant seals in less shark crowded waters, they tend to bite , disabling the flippers and wait for the creature to bleed out and be less dangerous. In the case of humans however, other people pull the dead or injured swimmer out of the water before the shark can feed.
They do in fact test bite, to see if am object is food, not to be fussy about what kind of food it may be. Nesting great white females will also kill when not hungry in they feel their nest area is threatened.