[Koha] .mrc file import issue

Hi, I am new to Koha. I am facing some problem regarding database. If I want to import a .mrc file after truncating biblio, biblioitems and items tables, then the biblionumber is increasing. For example, if I truncate 3000 rows from the above tables, then after impoting the .mrc file, it is starting from 3001. If you kindly help why I am not getting biblionumber 1 second time, then I will be grateful. -- Via Vitae Solutions Kolkata - 700064 India