arp replied with a mac, but the mac can not found in "brctl showmacs"

[DogLi]

A virtual machine connet to a bridge, which name is br_eth_nfs, and get the IP when boot up. It tells me someone other used the ip, which is 172.16.30.34. So I use command apping -c 5 -D -I br_eth_nfs 172.16.30.34 , and the get the reply from MAC address 90:E2:BA:7A:A1:99. Then I see the MAC table in bridge with command brctl showmacs br_eth_nfs, but can not find the MAC 90:E2:BA:7A:A1:99, and I can not find any other device connect to the birdge. How can I find the device with MAC 90:E2:BA:7A:A1:99?