Hibernate inserting two times in table using @TableGenerator

I am creating an Entity in Hibernate and using @TableGenerator for generating Primary key values. My Entity class is given below:

@Entity
@Table(name="studenttable")
public class Student {

@TableGenerator(
            name="stdGen",
            table="ID_GEN",
            pkColumnValue="studentId",
            pkColumnName="GEN_KEY",
            valueColumnName="GEN_VALUE", allocationSize = 10)
 @Id
  @GeneratedValue(strategy = GenerationType.TABLE, generator = "stdGen")
   private int studentId;
//rest of the code
}

My question is when I execute the code, initially two times insertion happens ID_GEN table. The console shows like this

Hibernate: select tbl.GEN_VALUE from ID_GEN tbl where tbl.GEN_KEY=? for update
Hibernate: insert into ID_GEN (GEN_KEY, GEN_VALUE)  values (?,?)
Hibernate: update ID_GEN set GEN_VALUE=?  where GEN_VALUE=? and GEN_KEY=?
Hibernate: select tbl.GEN_VALUE from ID_GEN tbl where tbl.GEN_KEY=? for update
Hibernate: update ID_GEN set GEN_VALUE=?  where GEN_VALUE=? and GEN_KEY=?

Please explain why it is happening like this. After that it is working fine. I am writing the below code to insert two objects in the table.

 Configuration cfg = new Configuration();
 cfg.configure();
 SessionFactory sessionFactory = cfg.buildSessionFactory();
 Session session = sessionFactory.openSession();
 session.beginTransaction();
 session.insert(new Student("ABC"));
 session.insert(new Student("DEF"));       
 session.getTransaction().commit();

Using Hibernate 5.2.3 version.