Why doesn't a variable change when assigned a function result?

Consider this:

function someIntFunction: integer;
 begin
   if false then
     result := 20;
 end;
 
 procedure someIntMethod;
 var
   lValue: integer;
 begin
   lValue := 10;
   lValue := someIntFunction;
   ShowMessage(IntToStr(lValue));
 end;

What does ShowMessage show?

When this code is compiled, a compiler warning is issued for someIntFunction indicating that the result may be undefined. This is a valid warning and what is displayed will be whatever happens to be on the stack; some random value.

Now consider this:

function someStringFunction: string;
 begin
   if false then
     result := '20';
 end;
 
 procedure someStringMethod;
 var
   lValue: string;
 begin
   lValue := '10';
   lValue := someStringFunction;
   ShowMessage(lValue);
 end;

When this is compiled, there is no warning for someStringFunction. A warning should probably also be emitted, but it's not. The effect however is a bit different. In this case, the variable that the result is assigned to is unchanged. This seems to occur for any type that is reference counted: strings, dynamic arrays and interfaces.