Unpredicatable sequence for reading logical file on V5R3

Have a customer who just moved from a model 730 at V5R2 to a model 520 at 
V5R3.  They've come across a situation where they have a logical file that 
on the old system always retrieves the records in a predictable 
chronological sequence (basically the order the records are in the 
physical file) even though the logical does not define this.  Now I know 
the old "unpredictable results may occur" caveat, but does anyone know why 
the records would be retrieved in a different/random sequence on V5R3 as 
opposed to V5R2 ?    Obviously the solution is really to define the 
logicals to explicitly force the correct sequence, but they are using a 
3rd party package and afraid they may miss some files.

...Neil