Why is it so? VOIP requires interface rule

[gwaitsi]

I have a VOIP setup over the OpenVPN and it works, but i don't understand why I need to add at the interface level.

NAT Port Forward
Src Add Src Ports Dest Add Dest Ports Nat Add Nat Ports Gateway

•          *              to    VOIPBOX   VOIP_PORTS      VOIPBOX       VOIP_PORTS        VPNGW
  

Rules
auto-created (in OpenVPN)
Src Add Src Ports Dest Add Dest Ports Gateway

•          *              to    VOIPBOX   VOIP_PORTS      VPNGW 
  

the above doesn't work, until I add the below
Rules
VPN1 & VPN2
Src Add Src Ports Dest Add Dest Ports Gateway

•          *              to    VOIPBOX   VOIP_PORTS      VPNGW 
  

Why do I need to add this to the VPN interfaces, instead of the OpenVPN. This is also true for allowing the pfsense to connect to the upgrade server. I have to create a rule on the VPN interfaces to allow VPN1 & 2 net to HTTPS or the packages are not available. If i create under OpenVPN it doesn't work.