It's very good question.
Generally AWS tries to keep packet as long as we can with in AWS network and try to exit it closest to destination. But there are circumstances where we need to exit out of AWS network quickly, possible reason could be.
More bandwidth availability on Local Exit ISP/ reduced latency/traffic manipulation during maintenances. But generally we try to keep traffic on AWS internal network (cold potato routing)as we have full visibility of Network path and AWS network is optimised for best performance but its not always the case.
Doing a trace route from your EC2 to your destination will give you some clue.
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