auto_increment value increased from 20 to 32 when I inserted a new row.

[crocket]

Below is the definition of the table with the problem.

CREATE TABLE `series` (
  `series_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `title` varchar(200) NOT NULL,
  PRIMARY KEY (`series_id`)
) ENGINE=InnoDB

I had 19 rows in series table. And when I tried inserting the 20th
row, the auto_increment value suddenly increased from 20 to 32, and
the new row has 20 as series_id.

This leap seems to happen on every table.

What is the cause of the leap in auto_increment?