sql count() get wrong result - mysql

Question

I have a table like this structure:
| event id | item 1 id | item 2 id | set |
| 1 | 1 | 2 | 1 |
| 1 | 1 | 3 | 1 |
| 1 | 2 | 1 | 1 |
| 1 | 2 | 3 | 1 |
| 1 | 3 | 1 | 1 |
| 1 | 3 | 2 | 1 |
| 1 | 2 | 4 | 2 |
| 1 | 4 | 2 | 2 |
| 2 | 1 | 4 | 3 |
| 2 | 1 | 5 | 3 |
| 2 | 4 | 1 | 3 |
| 2 | 4 | 5 | 3 |
| 2 | 5 | 1 | 3 |
| 2 | 5 | 4 | 3 |
now I wish to count the occurrence of item1 as well as item1 combined with item2 separately
I tried following:
with count_item1 AS (
select event_id, item_1_id, count(distinct set) AS c1 from table
group by event_id, item_1_id
), count_item1_and_item2 AS (
select event_id, item_1_id, item_2_id, count(distinct set) AS c2 from table
group by event_id, item_1_id, item_2_id
)
select t1.event_id, t1.item_1_id, t1.item_2_id, t1.c2, t2.c1
from count_item1_and_item2 AS t1
inner join count_item1 AS t2
on t1.event_id=t2.event_id and t1.item_1_id=t2.item_1_id
for example as table above
so the result for this should be:
| event id | item 1 id | item 2 id | c1 | c2 |
| 1 | 1 | 2 | 1 | 1 |
| 1 | 1 | 3 | 1 | 1 |
| 1 | 2 | 1 | 2 | 1 |
| 1 | 2 | 3 | 2 | 1 |
| 1 | 3 | 1 | 1 | 1 |
| 1 | 3 | 2 | 1 | 1 |
| 1 | 2 | 4 | 2 | 1 |
| 1 | 4 | 2 | 1 | 1 |
| 2 | 1 | 4 | 1 | 1 |
| 2 | 1 | 5 | 1 | 1 |
| 2 | 4 | 1 | 1 | 1 |
| 2 | 4 | 5 | 1 | 1 |
| 2 | 5 | 1 | 1 | 1 |
| 2 | 5 | 4 | 1 | 1 |
the meaning of each row is: in event_id, item 1 appeared c1 in distinct set and (item1, item2) appeared c2 in distinct set.
also count of (item1, item2) is equal to (item2, item1)
then I find a strange thing:
the occurrence of item1 itself should not less than the occurrence of item1 combined with item2, but I find that sometimes item1 combined with item2 has more count than count of item1, is there anything I did wrong here? I think my idea is right but I didn't get the result. I have being stacked in here for a weekend.