More likely the upstream packets are getting "backed up" in the upstream queue. These packets include ACKS to incoming packets. In this case the ping time is including the time the ping was stuck in the outgoing queue. This occurs even without a router.
If the packets were being dropped by the router, the pings would time out. -- "snmp: the standard e-mail protocol on the Internet" - LinkSys user manual (page 17)
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