I think this could be relevant. a LOT of devices drop snmp requests
when they get busy or when too many incoming requests occur. Are you
sure that you were the only one polling that device? Perhaps someone
else put it into a "busy" state. Too often with SNMP devices and tools
a '0' can mean things other than zero.
So you are saying that it's ok for a Cisco or Juniper router to return
zero for a counter when they feel "busy" ?