Update specific rows in a column
you are missing a key based on what you can update the data ... obviously there is always the last value in table1.my_col because what you are doing is looping through all values and updating the column in the table1..

The database design is not correct ..  Can you share your table structure / definition ?

Then, we can come up with some solution for you

EDIT:
if you wanna just move those records from one table to another ... 

try this:

insert into t1 (my_col) as select acct_num from t2 where rownum not in ('1', '10','38', '39', '46', '77');