Source port change

Dear F5 Experts, I'm trying to understand why F5 most often changes the source port when proxying connections towards Internet in our setup. We don't use any SNAT and so client source IP is preserved fine, however most often the F5 is using different port on the outbound connection. This is a Viprion CMP system with 12 TMMs per blade. The setting on the virtual server is "SourcePort=Preserve" but looks like F5 is for some reason forced to change this. I can't see how there can be a collision for ports here since the tuple:

client_ip:src_port - server_ip:dst_port

is always unique as the client_ip is unique. So even if there is a hash done one client_ip or src_port to always direct traffic from specific users or ports to same TMM for both flows client and server side, I'm still not sure why the port has to be changed on server side connection ?