I think you misunderstand the RFC.
Consider a ftp server on port 21 when a PORT command is ussed PORT 216,139,145,218,13,80 The FTP server connects to IP 216.139.145.218 PORT 3408. The connection will originate from port 20. Provided the ftp server is following the RFC.
The FTP server assigns the local port 1 below the control connection for its data connections. When the client issues a PORT command the connecting port can be any port within the range of 1025-65535.
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