Why optimize is not executed on all shards?

I tried to execute optimize operation to one index:

curl -XPOST '
http://192.168.2.105:9200/user/_optimize?max_num_segments=5&pretty=truehttp://192.168.2.105:9200/weibo_user_v1/_optimize?pretty=true
'
{
"ok" : true,
"_shards" : {
"total" : 10,
"successful" : 6,
"failed" : 0
}

Why only 6 shards are successful? (The index has 5 shards and 1 replica)

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