ACL protected flow service getting called with "Default" access

My flow service has exceute acl = ‘aaa’. This acl has certain number of users.

When my service is accessed from html page, it is prompted with uid/pwd. I enter uid/pwd.
Then I am seeing 2 entries in the audit log of the “Administrator” page.
1. call of my service with “Default” access. This fails.
2. call of my service with correct uid. This succeeds.

Why is this happening? Don’t you think it is the performance issue to call the same service first with “Default” uid and next with the entered uid?

Is there any parameter that we can use to tune, to avoid the above behavior?

Team,

Do anybody has any idea why 2 calls are made for the same flow service? Is there any workaround for this problem?

Have any idea?

Your help is appreciated.