postgresql – Expected function to return very few rows, but returns all rows

I am creating a Postgres/postgis function that is suppose to cut an input-geometry according to a ‘hard-coded’ table’s geometry. The hard-coded table is called j.borders and its geometry attribute is called geom. If the input-geometry intersects several geometries (rows) from j.borders, j.borders geometry is collected into one geometry (using st_union), which is then used to cut the input-geometry using st_difference. If the input-geometry doesn’t intersects j.borders geometry, the input-geometry is returned un-modified.

The function I have created behaves as i want it to, however:
I dont understand why it behaves like this

the function in question:

    IN in_geom geometry,
    OUT out_geom geometry)
-- can be called like SELECT public.clip_geom(geom) FROM a_table

  RETURNS geometry AS

    st_multi( -- st_multi makes sure the geometry is always the same "type"
    ST_DIFFERENCE(in_geom,st_union(brdr.geom)), -- cutting the geometry 
    in_geom -- not cutting the geometry
    INTO out_geom  
    FROM j.borders brdr
    WHERE ST_INTERSECTS(in_geom, brdr.geom); -- if the two geometries intersects 

  LANGUAGE plpgsql;

If you give this function a table of 5 rows with geometry and 2 of them intersects the border-geometry, it then returns all 5 rows, where 2 have been clipped to the border.

the select statement is followed by a where-clause which states the input-geometry and border-geometry have to intersect. If the two geometries do not intersect, nothing is returned, correct?
but why does the function then return the unmodified geometry? Does it have something to do with the COALESCE?