Overlapping subfields with a MOVE

A programmer here was making simple changes and happened to have a data
structure with overlapping sub fields.  

Example code:
     DDS               DS                            
     D FLD1                    1     10              
     D FLD2                    1     10              
                                                     
     C                   MOVE      FLD1          FLD2

The compiler complains about the above code but not this...
     C                   EVAL      FLD2 = FLD1

So, we were wondering what is significant about the two operations that
prevent MOVE from working over EVAL?  

I did search the archives and FAQ using "RNF7017" and "overlapping sub
fields" (and variations of) and I found nothing.  This is just a query
for a simple answer.  We know the solution is to change MOVE to EVAL
whenever possible and not to move data for overlapping sub fields.  


Thank you,
Matt Tyler
WinCo Foods, LLC
mattt@xxxxxxxxxxxxxx