The green highlighting fill doesn't mean the answer itself is accepted, it (usually) means that specific reputation gain was from an acceptance. Much the same as it shows in the full reputation tab.
As you can see here:
I assume a bounty award gets that highlighting fill applied to the summary, just as it does in the the full reputation history.
Basically, anything that has the filled background in the reputation tab also gets it in the summary. The only difference being that bounties don't get styled differently (i.e. blue vs green) in the summary—probably just an oversight (an old one, apparently).