GROUP BY MONTH gives wrong result

Question

I am trying to get unique products views grouped by month using the following query

SELECT
  Month(timestamp)           AS 'month',
  COUNT(DISTINCT visitor_id) AS 'unique'
FROM productviews pv INNER JOIN products p ON pv.product_id = p.id
WHERE p.vendor_id = 8 AND YEAR(timestamp) = 2018
GROUP BY month(timestamp);

And I am getting

+---------+--------+
| month   | unique |
+---------+--------+
|    1    |  3     |
+---------+--------+
|    2    |  10    |
+---------+--------+
|    3    |  2     |
+---------+--------+
|    4    |  4     |
+---------+--------+

But the total number of unique product views is less than the sum of the previous query

SELECT count(DISTINCT pv.visitor_id)
FROM productviews pv INNER JOIN products p ON pv.product_id = p.id
WHERE p.vendor_id = 8 AND year(timestamp) = 2018

+---------+
|  count  |
+---------+
|    16   |
+---------+

Is it the right way to use group by month? Or Do I miss something?

Answer 1

Your query is OK.

Because a visitor can have been visiting more than one month.