10+10=20. It shows 20 because you could get the same character twice in the rewards (10 shards for each time). It's trying to tell you that you'll receive 20 total character shards, and which characters shards you could get, while also showing the maximum number of shards you could get for any one character. Is it somewhat counter-intuitive? Yes. But once you know what it means it makes sense.
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Got the same thing going on
https://forums.galaxy-of-heroes.starwars.ea.com/discussion/comment/2196972#Comment_2196972