DAX use of Measure direct or as Variable

Hi
Workbook examples attached.

Can someone explain the difference as to why one works and one doesn't;

I'm adding an index to some values in power pivot using DAX, the formula

   OneV:=IF(HASONEVALUE(Table1[Test]),VALUES(Table1[Test]))

This is then used as var in ;

 OneV VAR:=VAR onev = [OneV]
                                       RETURN 
                                      CALCULATE([CountR],FILTER(ALL(Table1[Test]),
                                                                            ISONORAFTER(Table1[Test],onev,DESC)))

My question is that if I just use the measure directly, not as a variable first, i ceases to work?

Measure used  OneV:=CALCULATE([CountR],FILTER(ALL(Table1[Test]),ISONORAFTER(Table1[Test],[OneV],DESC)) )

if I type the 'if onevalue' into the formula it all works again, what's happening?

Richard.