Why does result change when using VAR vs MEASURE in another measure?

Yes, DAX is complex under the hood, but you shouldn't lose confidence using VAR.  VAR actually simplifies DAX a lot.  All you need to know is

1. VAR is evaluated first, before any changes are made to filter and row contexts.
2. A measure has a hidden CALCULATE, so if you are using VAR instead of a Measure, you need to wrap the raw formula inside a CALCULATE if you want to guarantee the same result

Here is an example of a formula where VAR makes it easier.

Consider the following calculated column.

Rank Customer by Age =
COUNTROWS (
    FILTER ( Customer, Customer[Birth Date] < EARLIER ( Customer[Birth Date] ) )
)

This formula has 2 row contexts, one from the calculated column and one from FILTER.  In order to make it work, you have to use the EARLIER function to refer to the outer row context.  Instead, you could use VAR here

Rank Customer =
VAR ThisCustomersAge = Customer[Birth Date]
RETURN
    COUNTROWS ( FILTER ( Customer, Customer[Birth Date] < ThisCustomersAge ) )

VAR gets evaluated first.  It assigns the customer's birthdate for each row in the customer table to the variable, then it filters the customer table to see how many customers are older.