Your master component has
DTSTART:20130411T050000
but your exception has
RECURRENCE-ID:20130413T053000
The recurrence id should always match one of (expanded) instances of the event. The date part is correct (20130413 is the saturday following the first instance) but the time part is 053000 when the first instance shows a time part of 050000.
In other words, your recurrence-id should be:
RECURRENCE-ID:20130413T050000