[n1ql ASC/DESC] Query is not using the same index consistenty.

Indexes created:
CREATE INDEX idx1 ON default(join_yr ASC, join_day DESC, VMs ASC)
CREATE INDEX idx2 ON default(join_yr DESC, join_day DESC, meta().id DESC)

Query issued:
explain SELECT * FROM default WHERE join_yr > 10 ORDER BY join_yr, join_day DESC LIMIT 10 OFFSET 20

This query uses idx1 mostly.

But intermittently it uses idx2.

Example document:
{
"tasks_points":

{ "task1": 1, "task2": 1 }

,
"name": "employee-9",
"mutated": 0,
"skills": [
"skill2010",
"skill2011"
],
"join_day": 9,
"join_mo": 10,
"email": "9-mail@couchbase.com",
"test_rate": 10.1,
"join_yr": 2011,
"_id": "query-testemployee10153.1877827-0",
"VMs": [

{ "RAM": 10, "os": "ubuntu", "name": "vm_10", "memory": 10 }

,

{ "RAM": 10, "os": "windows", "name": "vm_11", "memory": 10 }

],
"job_title": "Engineer"
}