Insert Increasing Auto Increment Value - mysql

Question

I'm having a problem with the auto increment id increasing when I don't want to it. I'm aware that the auto increment id increases when using INSERT IGNORE so I'm working around that, but still getting a behavior I can't figure out.
I'm building a normalized table of transactions, and in this table there is a column for first name which will have a reference table of transaction_first_names. My workflow is that I load data into a non normalized staging table, compare the values in the staging table with the values in the reference tables and if they do not exist into the reference table, then move the data from the staging table to the normalized table.
The issue I'm having is that when I try to insert any "new" values from the staging table into the reference tables, it seem to increment the autoincrement id's in the reference table in a way I can't explain. I wouldn't normally be ocd or stingy with id's, but as a continuing process I don't want the id's to continually be chewed through.
Here is my setup, link & code. As you can see in the second result the last inserted value was given the id of 16, whereas the goal is that id should be 9:
Runnable Example - http://rextester.com/KVMO89341
CREATE TABLE IF NOT EXISTS `transaction_first_names` (
`id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`first_name` VARCHAR(100) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE INDEX `u_first_name` (`first_name`)
)
COLLATE='utf8mb4_general_ci'
ENGINE=InnoDB;
CREATE TABLE IF NOT EXISTS `transaction_stage` (
`transaction_id` BIGINT(20) UNSIGNED NOT NULL,
`first_name` VARCHAR(255) NULL DEFAULT NULL,
PRIMARY KEY (`transaction_id`),
INDEX `first_name` (`first_name`(191))
)
COLLATE='utf8mb4_general_ci'
ENGINE=InnoDB;
TRUNCATE transaction_stage;
TRUNCATE transaction_first_names;
INSERT INTO `transaction_stage` (`transaction_id`, `first_name`) VALUES (3658822144, 'Michael');
INSERT INTO `transaction_stage` (`transaction_id`, `first_name`) VALUES (3658825319, 'Pete');
INSERT INTO `transaction_stage` (`transaction_id`, `first_name`) VALUES (3658828867, 'Robert');
INSERT INTO `transaction_stage` (`transaction_id`, `first_name`) VALUES (3658865656, 'Martin');
INSERT INTO `transaction_stage` (`transaction_id`, `first_name`) VALUES (3659080925, 'Charlews');
INSERT INTO `transaction_stage` (`transaction_id`, `first_name`) VALUES (3659943769, 'Christopher');
INSERT INTO `transaction_stage` (`transaction_id`, `first_name`) VALUES (3660191699, 'Robert');
INSERT INTO `transaction_stage` (`transaction_id`, `first_name`) VALUES (3660192662, 'Errol');
INSERT INTO `transaction_stage` (`transaction_id`, `first_name`) VALUES (3660194469, 'Frank');
INSERT INTO `transaction_stage` (`transaction_id`, `first_name`) VALUES (3660200483, 'Frank');
-- first select
SELECT DISTINCT st.first_name
FROM transaction_stage st
LEFT JOIN transaction_first_names f ON st.first_name <=> f.first_name
WHERE f.id IS NULL
AND st.first_name IS NOT NULL;
-- first insert
INSERT INTO transaction_first_names (`first_name`)
SELECT DISTINCT st.first_name
FROM transaction_stage st
LEFT JOIN transaction_first_names f ON st.first_name <=> f.first_name
WHERE f.id IS NULL
AND st.first_name IS NOT NULL;
-- second insert
INSERT INTO transaction_first_names (`first_name`)
VALUES ('Another name');
-- check autoincrement
SELECT * FROM transaction_first_names order by id asc;
DROP TABLE IF EXISTS transaction_first_names;
DROP TABLE IF EXISTS transaction_stage;
I've tried wrapping the select distinct in the first insert statement, but no luck.

Answer 1

Ah, InnoDB handles things a bit differently depending on how the system variable innodb_autoinc_lock_mode is set.
For lock modes 1 or 2, gaps may occur between successive statements
because for bulk inserts the exact number of auto-increment values
required by each statement may not be known and overestimation is
possible.
https://dev.mysql.com/doc/refman/5.7/en/innodb-auto-increment-handling.html