DAX RANKX ; Using Measure versus actual expression

[dicken]

Hi,
In MS DOCS on RANKX it has the following;

=

RANKX(ALL(Products), SUMX(RELATEDTABLE(InternetSales), [SalesAmount]))

But on trying this myself I could not get this to work;

RankItemC:=RANKX( ALL(Table1[Item]),  SUM(Table1[Unit]), ,DESC ,Dense )

But if I Used the Measure TotalUnits ; SUM(Table1[Unit]),

RankItemA:=RANKX( ALL(Table1[Item]),[TotalUnits] ,,DESC,Dense)

Then it did, likewise if I wrapped the SUM in Calculate ;

RANKX( ALL(Table1[Item]), CALCULATE( SUM(Table1[Unit])),,DESC ,Dense )

It worked, my question regards why the initial MS example does not need calculate?

Richard