User attributes maps

Hi! I’m still reading about Keycloak and learning a lot with this great tool, I’m using the last version of Keycloak and building a service that consumes Keycloak admin’s API, when I create a User, I do it with the following data
{ "username": "user1", "firstName": "user", "lastName": "user", "email": "", "attributes": { "attr1": "value1" } }

And when I call the endpoint http://localhost:5000/auth/admin/realms/{realm}/users?username=user1

the answer that I recieve is
[ { "id": "e741b717-4aca-45e6-a10a-9705991f9806", "createdTimestamp": 1596059762402, "username": "user1", "enabled": false, "totp": false, "emailVerified": false, "firstName": "user", "lastName": "user", "email": "", "attributes": { "attr1": [ "value1" ] }, ... } ]

with this answer the key “attributes” is an object that has a key and that key is an array, is this a normal behavior?
Sorry if my English is not very good!