problem in table control

hi experts,

i have declared a table is_ftable LIKE TABLE OF i_itf WITH HEADER LINE

DATA: BEGIN OF i_itf OCCURS 0,

vselect TYPE c,

tabname TYPE dfies-tabname,

fieldname TYPE dfies-fieldname,

fieldtext TYPE dfies-fieldtext,

position TYPE dfies-position,

END OF i_itf.

after populating is_ftable, i am outputting its content on table control.on executing first time there are 21 rows outputted.

on scrolling down the is_ftable-position field from position 1 to 21 changes to '21'.

why is content of only position field getting changed rest of the field remains same?

thanks.