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This is the way InDesign works with JavaScript. For performance 
reasons, a variable containing an object stores a specifier to an 
object rather than the object itself. You can see what the specifier 
looks like using the "toSpecifier()" method.

Accessing an object from a collection by subscript (as you are 
doing), creates a specifier that is relative to the location in the 
collection (as you put it, the variable is assigned to the array 
location). If the contents of the collection changes, then the 
specifier may refer to a different object.

Whether creating a new object changes the contents of the collection 
depends on the type of object. For example, items in a page item 
collection (including text frames) are returned from front-to-back in 
the order the objects appear on the page. Since a new page item is 
created on top by default, the newest page item will appear at the 
beginning of the collection. (An example of an object that is added 
to the end of its collection by default is a spread.)

Most objects (for example, those that have a unique id) can be forced 
to self-identify uniquely using the "getElements()" method, which 
retrieves (an array of) the object(s) referred to by a specifier at 
the time the method is invoked. Keep in mind, however, that some 
objects always use relative specifiers (e.g., text).

Here's a script to see the difference between relative and unique specifiers:

var doc = app.documents.add() ;

//tfA is the unique specifier of a new text frame (A)
var tfA = doc.textFrames.add() ;
alert( "tfA = " + tfA.toSpecifier() ) ;
//tfFIRST is a specifier to the first item in the collection
var tfFIRST = doc.textFrames[0] ;
alert( "tfFIRST = " + tfFIRST.toSpecifier() ) ;
//tfFIRST_A is the unique specifier of the first item in the collection (A)
var tfFIRST_A = tfFIRST.getElements()[0] ;
alert( "tfFIRST_A = " + tfFIRST_A.toSpecifier() ) ;

//tfB is the unique specifier of a new text frame (B)
var tfB = doc.textFrames.add() ;
alert( "tfB = " + tfB.toSpecifier() ) ;
//tfFIRST is still a specifier to the first item in the collection
alert( "tfFIRST = " + tfFIRST.toSpecifier() ) ;
//tfFIRST_B is the unique specifier of the first item in the collection (B)
var tfFIRST_B = tfFIRST.getElements()[0] ;
alert( "tfFIRST_B = " + tfFIRST_B.toSpecifier() ) ;
//tfSECOND is a specifier to the second item in the collection
var tfSECOND = doc.textFrames[1] ;
alert( "tfSECOND = " + tfSECOND.toSpecifier() ) ;
//tfSECOND_A is the unique specifier of the second item in the collection (A)
var tfSECOND_A = tfSECOND.getElements()[0] ;
alert( "tfSECOND_A = " + tfSECOND_A.toSpecifier() ) ;

//tfALL is a specifier to all items in the collection
var tfALL = doc.textFrames.everyItem() ;
alert( "tfALL = " + tfALL.toSpecifier() ) ;
//tfALL_B is the unique specifier of the first item in the collection (B)
var tfALL_B = tfALL.getElements()[0] ;
alert( "tfALL_B = " + tfALL_B.toSpecifier() ) ;
//tfALL_A is the unique specifier of the second item in the collection (A)
var tfALL_A = tfALL.getElements()[1] ;
alert( "tfALL_A = " + tfALL_A.toSpecifier() ) ;