Union query

I have two queries as follows query-1 ------- select tabid,"from_1st_select" from systables where tabid < 4 union select tabid,"from_2nd_select" from systables where tabid < 4 query-1-output -------------- tabid (constant) 1 from_1st_select 1 from_2nd_select 2 from_1st_select 2 from_2nd_select 3 from_1st_select 3 from_2nd_select query-2 ------- select "from_1st_select" from systables where tabid < 4 union select "from_2nd_select" from systables where tabid < 4 query-2-output -------------- (constant) from_1st_select from_2nd_select Question : the 1st query returns 6 rows, that is ok, why 2nd query is returning 2 rows, why not 6 rows