MySQL returning multiple rows that does not match search criteria

I have MySQL installed locally, running SELECT VERSION() returns this value: 5.6.43-84.3

When I run a query it is returning multiple rows when it should only return 1 row. Let me set it up, it's easier to explain that way.

1. Create a test table:

    CREATE TABLE test_table 
    (
        test_val VARCHAR(255)
    )
    ;

2. Load 3 values into the table:

    INSERT INTO test_table (test_val)
    VALUES 
    ('9671986020630615'),
    ('9671986020630616'),
    ('9671986020630617')
    ;

3. Run this query (This query returns 1 row which is expected):

    SELECT * 
    FROM test_table
    WHERE test_val = '9671986020630615'
    ;

4. Run this query (This query returns 3 rows, which it shouldn't):

    SELECT * 
    FROM test_table
    WHERE test_val = 9671986020630615
    ;

Here's what I have observed about this situation:

1. The first query surrounds the value in the WHERE clause with single quotes.
2. The second query does not surround the value in the WHERE clause with single tics.
3. The column in the test table is defined as VARCHAR(255)
4. It makes sense that the first query returns just one row because it's comparing a string from the WHERE clause to a string value in the test table (VARCHAR(255))
5. Something is happening when MySQL compares the numerical value in the WHERE clause of the second query to the string value in the test table (VARCHAR(255)) which is causing MySQL to return 3 rows instead of just 1.

It makes sense that the first query returns the correct result because it is comparing a string to a string.

It also makes a degree of sense that the second query is returning a bad dataset (3 rows as opposed to the 1 row it should return).

But my question is why is MySQL doing this? Why when it compares a number to 3 different VARCHAR(255) values does it return all 3 rows when the true value of the numerical value in the WHERE clause only matches 1 row?

So, in essence for the first query MySQL is saying:

'9671986020630615' = '9671986020630615',

'9671986020630615' <> '9671986020630616',

'9671986020630615' <> '9671986020630617'

but for the second query it is saying:

9671986020630615 = '9671986020630615',

9671986020630615 = '9671986020630616',

9671986020630615 = '9671986020630617'

Any help will be much appreciated.