fluctuations in generation of primary key auto increment value. (no replies)

Dear friends,

I was just try to Insert some columns in mysql table.table consists primary key with auto increment (seed value=1).

So Intitially table is :

id name
---------------
1 williams
2 andrew

But After i inserted,the table is :

id name
---------------
1 williams
2 andrew
4 methew
5 ram

So i am not aware why some fluctuations happening in autoincrement value.Why the id 3 was ignored.
So please clarify my doubt.

Thanks in advance.