Using twurl to hit the API endpoint I get 60 RTs; I can easily believe that the remainder are from protected accounts, which the API wouldn’t return. Can you reproduce this?
[tw-mbp-isaac ~]$ twurl "/1.1/statuses/retweets/521593287422251008.json?trim_user=true&count=100" | python -mjson.tool | grep -c "retweeted_status"
60