Connection Mirroring with connections showing on Standby

Yes, that's completely normal. This information is kept in the table to make the Standby unit a "hot spare". In an event of a fail over, your Standby Unit is ready to resume existing stateful connections mid-stream. Without connection mirroring, your BigIP would respond to any alien TCP/ACK or TCP/PSH packets with a TCP/RST packet.

Note that in case of HTTP-based services or other protocols that have built-in retry mechanism in L7, the connection mirroring feature is unlikely to add any value.