Why wireshark capture RTP when portmirroring? why doesn't WS capture RTP if not in portmirroring??

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Hello, I set simple VoIP testbed.

1 Hub connected with 4 PCs (1 SIP Server, 2 SIP UserAgent, 1 Sniffing Computer)

I installed Wireshark in Server PC and Sniffing Computer.

I did portmirroring between 1 SIP UserAgent and 1 Sniffing Computer.

And I start VoIP Call between UAs.

Wireshark in Sniffing Computer captured RTP, SIP/SDP, UDP packets.

but Server PC did not capture RTP packets but UDP, SIP/SDP.

why did not Server PC capture RTP packet????