Forwarding VIP or default route

Default route & Forwarding VIP are different.

Forwarding VIP: IP forwarding VIP accepts traffic that matches the VIP and forwards it to the destination IP address that is specified in the request rather than LB the traffic to a pool. NAT/SNAT is disabled when you create an IP forwarding VIP, leaving the destination address in the packet unchanged. Go through article

Default Route: It is a set of standard routes for each route domain on the BIG-IP system. This route uses for the LB pool subnet if gateway didn't match with existing route(which is defined) for POOL VLAN provision subnet.