[CentOS] rup on CentOS6

I just noticed an interesting behaviour of rup on CentOS5/6: run without any
host args, it never prints results for CentOS6 machines. However, if a CentOS6
machine is queried directly, there is a result. It doesnt't matter whether the
querying host runs CentOS5 or CentOS6.

# rup |grep centos6host
# rup centos6host
centos6host            up  12 days, 19:33,    load average: 0.22 0.18 0.11
# 

With tcpdump, I can see that no rpc requests are issued to the host in the
first case.

Is there an explanation?