One side of the connection will have an arbitrary port number, the other will be on 179.
Cisco Press "BGP Fundamentals" has a good explanation (link)
the neighbor with the higher IP address manages the connection. The router initiating the request uses a dynamic source port, but the destination port is always 179.
Example 1-1 shows an established BGP session using the command
show tcp brief to display the active TCP sessions between routers. Notice that the TCP source port is 179 and the destination port is 59884 on R1, and the ports are opposite on R2.
Example 1-1: Established BGP session
RP/0/0/CPU0:R1# show tcp brief | exc "LISTEN|CLOSED"
PCB VRF-ID Recv-Q Send-Q Local Address Foreign Address State
0x088bcbb8 0x60000000 0 0 10.1.12.1:179 10.1.12.2:59884 ESTAB
R2# show tcp brief
TCB Local Address Foreign Address (state)
EF153B88 10.1.12.2. 59884 10.1.12.1.179 ESTAB
This is just the same as any other TCP connection: the passive open side sits and waits on a well-known port number; the active open side uses an arbitary port. This makes it much easier to manage many-to-many TCP links.