Oracle Database Discussions

Hi ,
I bumped to the following :

SQL> create table t ( x number, y number );
 
Table created
 
SQL> 
SQL> insert into t ( x )
  2   select to_number(rpad('9',rownum*2,'9'))
  3   from all_objects
  4   where rownum <= 14;
 
14 rows inserted

SQL> update t set y=x+1;
 
14 rows updated

SQL> select x ,y ,  vsize(x) , vsize(y) from t;
 
         X          Y   VSIZE(X)   VSIZE(Y)
---------- ---------- ---------- ----------
        99        100          2          2
      9999      10000          3          2
    999999    1000000          4          2
  99999999  100000000          5          2
9999999999 1000000000          6          2
9999999999 1000000000          7          2
9999999999 1000000000          8          2
      1E16       1E16          9          2
      1E18       1E18         10          2
      1E20       1E20         11          2
      1E22       1E22         12          2
      1E24       1E24         13          2
      1E26       1E26         14          2
      1E28       1E28         15          2
 
14 rows selected

Can somebody explain the above phenomenon where the vsize of x column gets incremented as the x number gets bigger.... but at the same time the vsize of y column gets the 'static' value of 2 whereas it is just bigger than x (x=9999 , y=10000)...????

The db version is 10.2.0.1.
Thanks...
Sim