Something reseeded your identity. Check this sample:

CREATE TABLE TestA (id INT NOT NULL IDENTITY(1,1) PRIMARY KEY , val VARCHAR(10));

insert TestA values ('a')

insert TestA values ('b')

insert TestA values ('c')

insert TestA values ('d')

insert TestA values ('e')

delete TestA where id < 4

DBCC CHECKIDENT ('TestA', RESEED, 0);

insert TestA values ('aa')

insert TestA values ('bb')

insert TestA values ('cc')

-- fine so far...

-- here is Oops!

insert TestA values ('dd')