Hi Santosh,
UNION ALL do not remove duplicates, what have this power on your scenario is the calculated view as Lars pointed out to you:
"The default for calculation view is to deliver unique records anyhow, as there is an implicit grouping, when at least one key figure/measure is defined."
About UNION to UNION ALL (on SQL as CE don't have UNION option) is that UNION provoke a row store transformation to DISTINCT the values removing the duplicates, and the UNION ALL clause just stack both results on column store. UNION ALL is faster but may produce duplicates as there's no distinct.
So it's working because you aren't summing up any measure, if you have it the result for values which exist on both projections both projections (LEFT OUTER & RIGHT OUTER) will be represented with double values.
Regards, Fernando Da Rós