Relationship not returning the same values as JOIN

Hey everyone,

I have a table which is in a relationship with a dimension table and I do the following calculation:

temp = IF(RELATED(Table2[Column1])=5, Table1[Column1])

Measure = DISTINCTCOUNT(Table1[temp])

However if I make a left join with the source table and the dimesnion table and change the calculations like this I get a different result:

temp = IF(Table[Column2])=5, Table[Column])

Measure = DISTINCTCOUNT(Table1[temp]) 

What is the difference between a relationship and a join? Why doesn't it work the same?