Full outer join in Measure's , IDT re Universe

Experts,

I'm trying to duplicate one measure in an universe but when I try to create report based on these measure's, it gives two query results.

Can anyone assist, why its creating two queries ( full outer join in query wizards in Information Design Tool and Union in Web Intelligence ).

For example,

measure 1:

CustomerCount :  Sum(fact_customer.CustomerCount)

Measure 2:

CustomerCount_by_city :  Sum(fact_customer.CustomerCount) where CustomerCity = 'Boston'

when I pull these two measures along with Customer name , it creates two queries and makes it join by using Union operator.

However If I drop the where condition from 'measure 2' it doesn't create two queries.

Thoughts ?

Thanks