Join / Inner Join inconsistency

Hi experts!

Select join is a statement in which it is recommended to use when accessing tables on header and item level. It is said that it will optimize database access but i am encountering a problem in which my where clauses don't actually get the correct data.

My code is below:

SELECT-OPTIONS: s_fkdat FOR likp-fkdat OBLIGATORY,

                s_vstel FOR likp-vstel,

                s_vbeln FOR likp-vbeln,

                s_tknum FOR vttk-tknum,

                s_knkli FOR likp-knkli.

    SELECT a~vbeln

           a~fkdat

           a~vstel

           a~knkli

           a~kdgrp

           b~vgbel

           b~lgmng

           b~aufnr

           b~pstyv

           b~charg

      INTO CORRESPONDING FIELDS OF TABLE it_likp

      FROM likp AS a

      INNER JOIN  lips AS b ON a~vbeln EQ b~vbeln

      WHERE a~fkdat IN s_fkdat

      AND   a~vstel IN s_vstel

      AND   a~vbeln IN s_vbeln

      AND   a~knkli IN s_knkli

      AND   a~kdgrp EQ '01'

      AND   b~vbeln IN s_vbeln

      AND   b~pstyv EQ 'KLN'

      OR     b~pstyv EQ 'ZANN'.

It doesn't filter the correct date (s_fkdat) when inputted on the selection screen. For example i inputed the date as this month, it still gets data from other months as supposed to only this month. As far as I know, a lot of people have been encountering this problem. What is the cause for this?

Thanks,

Alex