Mysql - The query left attached does not return the expected result

I have two query 1) tree and 2) units

first query

SELECT * FROM tree WHERE entity_id = 8656

return below data:

entity id | parent_id | length
-------------------------------
8656      | 8656      | 0
8656      | 8655      | 1
8656      | 8654      | 2
8656      | 8653      | 3
8656      | 4331      | 4
8656      | 2         | 5
8656      | 1         | 6

second query

SELECT * FROM units WHERE activity_id =10066

returns

id  | activity_id | activity_name | region_id | region_name
-----------------------------------------------------------
136 | 10066       | Cricket       | 4331      | Yote
137 | 10066       | Cricket       | 8653      | Handbreath
140 | 10066       | Cricket       | 8656      | Kevb

result expected

    entity id | parent_id | length | region_name
    --------------------------------------------
    8656      | 8656      | 0      | Kevb
    8656      | 8655      | 1      | null
    8656      | 8654      | 2      | null
    8656      | 8653      | 3      | Handbreath
    8656      | 4331      | 4      | Yote
    8656      | 2         | 5      | null
    8656      | 1         | 6      | null

I have tried this query

SELECT tree . * , units.region_name
FROM tree
LEFT JOIN units ON tree.parent_id = units.region_id
WHERE tree.entity_id = 8656
AND units.activity_id = 10066

---

By moving the u.activity_id =10066 to the LEFT JOIN and not the WHERE clause, I am able to produce the result:

select t.entity_id,
  t.parent_id,
  t.length,
  u.region_name
from tree t
left join units u
  on t.parent_id = u.region_id
  and u.activity_id =10066
WHERE t.entity_id = 8656

See SQL Fiddle with Demo