Cannot use parameters normally IIdentityUserAppService client proxy

use Normal IIdentityUserLookupAppService client proxy

string username="123"
IIdentityUserLookupAppService.FindByUserNameAsync(username);

Sending HTTP request GET https://localhost:44329/api/identity/users/lookup/by-username/123?api-version=1.0

string username="123"
IIdentityUserAppService.FindByUsernameAsync(username)

Start processing HTTP request GET https://localhost:44329/api/identity/users/by-username/{userName}?api-version=1.0

why IIdentityUserAppService.FindByUsernameAsync {userName}！= 123 ？