how xml path concatenation trick works

select * from

(select  temp.data + ''  from

 (

select  'aa' joindata , null data

union select 'ba' joindata , 'a' data

union select 'bb' joindata , 'a' data

union select 'bb' joindata , 'b' data

union select 'bc' joindata , 'b' data

 ) temp

where temp.data is not null

FOR XML PATH('')

) as results(result)

returns aabb

why?since in the sql the column is thus temp.data + ''  designated - sql server does not know how to tag the data and therefore leaves it empty