Pending Lifetime Processing

How is the determination of which messages to delete made?  I set 

PENDING_LIFETIME = "1d"

and then watched the list of pending messages.  After 24 hours a few 
more messages arrived and it removed several that were over 24 hours 
old.  About 20 minutes later I looked again and it had removed all but 
the last 8 minutes of messages from pending.  There were over 600 
messages in pending that were pretty evenly spread out throughout the 
hour and 20 minute period.  Is there a difference in the deletion if I 
specify:

PENDING_LIFETIME = "24h"