different result when query hint hint different indexes

//collection indexes

> db.Collection.getIndexes()> db.Collection.getIndexes()

[ 

... 

{ "v" : 2, "unique" : true, "key" : { "user" : 1, "word" : 1 }, "name" : "user_1_word_1", "ns" : "sample.Collection", "background" : true }, 

{ "v" : 2, "key" : { "user.$id" : 1, "word" : 1, "image" : 1 }, "name" : "user.$id_1_word_1_image_1", "ns" : "sample.Collection", "background" : true }]

// storeage Engine: wiredTiger

> db.Collection.find({"user" : DBRef("_User", ObjectId("5a5d783bee920a005896c55f"))},{"_id":1,"user":1}).hint("user_1_word_1").count()

39

> db.Collection.find({"user" : DBRef("_User", ObjectId("5a5d783bee920a005896c55f"))},{"_id":1,"user":1}).hint("user.$id_1_word_1_image_1").count()

8

// download attachments to $data_full_path

// md5: a14368dbab317182a4826caa0e7bc41d  sample.tgz

// pull mongo images

docker pull mongo:3.4.0-rc

docker run -v $data_full_path:/x mongo:4.0-rc  --setParameter failIndexKeyTooLong=false

// another terminal in docker

mongorestore --db sample --dir=./sample

// then query

mongo 127.0.0.1:27017/sample --eval 'db.Collection.find({"user" : DBRef("_User", ObjectId("5a5d783bee920a005896c55f"))},{"_id":1,"user":1}).hint("user_1_word_1").count()'

// result: 39

mongo 127.0.0.1:27017/sample --eval 'db.Collection.find({"user" : DBRef("_User", ObjectId("5a5d783bee920a005896c55f"))},{"_id":1,"user":1}).hint("user.$id_1_word_1_image_1").count()'

// result: 8