autoincrement column issue in MYSQL
The actual thing is the variable "innodb_autoinc_lock_mode" which control the behavior for auto_inc lock during generation of auto generated numbers. It can be set to three values which are 0 ,1 or 2. 

The catch is the behavior while setting this, if you set it to 0 which ensures that gap is not produced it held the table level lock and parallel inserts are disallowed while if you set it to 1 or 2 it ensure the lock is not held at table level and it is recommended to be set to 2 as it is fastest and allows parallel insert.

Just to add there may be gaps if there are some deleted rows and auto increment ensures the uniqueness not sequence. 

Hope it helps